TM 11-490-5

TECHNICAL MANUAL

ARMY COMMUNICATIONS FACILITIES

OPERATIONAL

ELECTROMAGNETIC

COMPATIBILITY

HEADQUARTERS, DEPARTMENT OF THE ARMYMARCH1976

TM 11-490-5

TECHNICAL MANUALHEADQUARTERS

DEPARTMENT OF THE ARMYNo. 11-490-5 WASHINGTON, DC, 30 March 1976

ARMY COMMUNICATIONS FACILITIES

OPERATIONAL ELECTROMAGNETIC COMPATIBILITY

Paragraph PageCHAPTER 1. INTRODUCTION.

Purpose................................................................................................ 1-1 1-1Scope................................................................................................... 1-2 1-1Definitions ............................................................................................ 1-3 1-1Recommendations for Changes ........................................................... 1-4 1-1

2. TYPES AND CAUSES OF EMC PROBLEMSGeneral ................................................................................................ 2-1 2-1Interference Sources ............................................................................ 2-2 2-1Transfer Media ..................................................................................... 2-3 2-10Equipment Susceptibility . ................................................................... 2-4 2-12

3. EMC PROBLEMS AS RELATED TO SIGNAL TYPES AND MULTIPLEXINGTECHNIQUES

General ................................................................................................ 3-1 3-1Signal Types and Multiplexing Techniques .......................................... 3-2 3-1

4. DETECTION, RECOGNITION, ISOLATION, AND REMEDY OF EMC PROBLEMSGeneral ................................................................................................ 4-1 4-1Discussion of EMC Problems................................................................ 4-2 4-1Recognition, Isolation, and Remedy of EMC Problems ........................ 4-3 4-3

5. PROCEDURES FOR REPORTING OPERATIONAL EMC PROBLEMSGeneral ................................................................................................ 5-1 5-1Procedures ........................................................................................... 5-2 5-1

APPENDIX DEFINITIONS A-1

INDEX .......................................................................................................... Index 1

}

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LIST OF ILLUSTRATIONS

Figure Page2-1 Typical Interference Sources ..................................................................................................................... 2-22-2 Transmitter Spurious Emissions for an AN/GRT-22 .................................................................................... 2-42-3 Example of Distortion as a Result of Overdriving an Amplifier Stage ......................................................... 2-52-4 Intermodulation Products Occurring at a Receiver ..................................................................................... 2-72-5 Intermodulation Products Occurring at a Transmitter ................................................................................. 2-82-6 Interference Signal Entry Paths ............................................................................................................... 2-122-7 Cochannel Interference ........................................................................................................................... 2-132-8 Adjacent-Channel Interference ................................................................................................................. 2-132-9 Intermediate-Frequency Interference....................................................................................................... .2-133-1 Typical Analog and Digital Signals ............................................................................................................. 3-13-2 Example of Analog Signal With Random Noise Interference ..................................................................... 3-23-3 Conversion of Analog to Digital Signals ..................................................................................................... 3-33-4 Example of Digital Signal With Interference ............................................................................................. .3-43-5 Examples of Channel Distribution of FDM and TDM .................................................................................. 3-54-1 Audio Signals Subjected to Various Interference Effects ............................................................................ 4-64-2 Examples of Oscilloscope Traces of 1 kHz Test Tone Signal to White Noise Interference Levels .............. 4-84-3 Examples of Oscilloscope Traces of an Analog Signal With Various Types of Interference ....................... 4-9

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TM 11-490-5

CHAPTER 1

INTRODUCTION

1-1. PurposeThis manual is intended to provide guidance tooperators of Communications-Electronics (C-E)equipments in identifying and resolving ElectromagneticCompatibility (EMC) problems. Guidance is providedfor requesting outside assistance when EMC problemscannot be solved with in-house resources.

1-2. ScopeThe procedures contained in this manual are to bestandard practices for US Army station and facilitycommunications operating personnel (technicalcontrollers, repairmen, and attendants). Advice andguidance are provided regarding identification andresolution of EMC problems and methods to be used forreporting and submission of requests for assistancefrom technical support organizations.

1-3. DefinitionsRefer to appendix.

1-4. Recommendations for Changes Users of this manual are encouraged to submitrecommended changes or comments to improve thepublication. Comments should be keyed to the specificpage, paragraph, and line of the text in which thechange is recommended. Reasons should be providedfor each comment to ensure understanding andcomplete evaluation. Comments should be prepared onDA form 2028 (Recommended Changes to Publicationsand Blank Forms) and submitted directly to theCommander, US Army Communications Command,ATTN: CC-POA, Fort Huachuca, Arizona 85613

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TM 11-490-5

CHAPTER 2

TYPES AND CAUSES OF EMC PROBLEMS

2-1. Generala. Electromagnetic interference (EMI), commonly

called "interference," is defined herein as anyelectromagnetic energy which degrades theperformance of C-E equipment.

b. EMC problems resulting from interference aredirectly related to three factors: (1) interference sources,(2) transfer media, and (3) equipment susceptibility.

2-2. Interference Sourcesa. General. Interference may result from either

natural or manmade causes (fig 2-1). While the effectsof interference from natural sources are geographicallywidespread, manmade interference (intentional orunintentional) usually affects only limited areas near thespecific sources. An exception is high frequency (HF)interference which may emanate from distant radiationswhich have been refracted from the ionosphere.

b. Natural Interference Sources. These sourcesmay be as close to the operator as his own equipmentand the earth's atmosphere, or as distant as the sun andthe outer reaches of the galaxy.

(1) Equipment noise. This noise, self-generated within electronic equipment, is generally

traceable to either "thermal agitation" or "shot effect."Thermal-agitation noise is caused by the unequal flow ofelectrons across a circuit element from the effects ofheat, resulting in a net voltage of random variation.Shot effect is caused by the random emission ofelectrons from an electron tube cathode. Noise in asemiconductor device is primarily caused by the randommotion of majority carriers crossing a junction and byfluctuations in the relative amounts of current flowingfrom the emitter to the collector and base of a transistor.The noise power level of a semiconductor device isgreater at the lower frequencies and depends upon thematerial of the semiconductors used. The types ofnoise described are strictly-speaking man-made,because the equipment is man-made. However, theyare described here as "natural" to make a point. Theydo-not arise from fault conditions or from equipmentmaladjustment or any other conditions which can beremedied by operating personnel. Nor are theyintentionally generated. The types of noise describedare "natural" in the sense that they are always present tovarying degrees in all energized electronic equipmentand the extent to which they can be minimized inpractice, is dependent on the funds available during thedesign and construction stages of the equipment.

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TM 11-490-5

Figure 2-1. Typical interferences sources.

(2) Atmospheric noise. The primary source ofatmospheric noise is lightning discharge inthunderstorms, which may result in a type of radiointerference commonly referred to as static. Anadditional source of atmospheric noise is precipitationstatic, produced by rain, hail, snow, or dust storms.Atmospheric effects are erratic in character, consistingof short, randomly recurring bursts of noise which lackany regularity in phase or amplitude and which arespread throughout the RF spectrum. The averagepower level of atmospheric noise is relatively constantover short periods of time, but may vary considerablyover longer periods of time. Atmospheric noise levelsdepend upon such factors as frequency, time of day,season, equipment location, and weather conditions.Atmospheric noise becomes significant at frequenciesbelow 50 MHz, and dominates all other naturalinterference sources below 30 MHz. This noise isusually the limiting factor in communications below'30MHz.

(3) Solar noise. Solar noise increases during

periods of high sunspot or solar flare activity anddecreases during periods of low activity. The periods ofhigh activity may last several days at a time, and noiselevels may be 10 to 20 dB greater than the levelsassociated with low activity. During a sunspot or solarflare activity, noise will first be noticed at the higher endof the high frequency (HF) region, then at the lower endas the disturbance matures. At times, the RF spectrumof these outbursts is quite wide and involves a majorportion of the radio frequencies now in use.

(4) Galactic noise. Galactic noise originatesbeyond the solar system from a number of sourceswithin the heart of the galaxy; that is, the Milky Way.Galactic noise intensity varies with frequency andposition within the galaxy, being strongest in thedirection of the galactic center (the constellationSagittarius). On the surface of the earth, galactic noiseis normally confined to ultrahigh frequency (UHF) andvery high frequency (VHF) regions.

c. Manmade Interference Sources. Manmade

2-2

TM 11-490-5interference is most prevalent near densely populatedareas and industrial districts, or within any other regionwhere many transmitters and other interference-producing equipments are operating simultaneously.Also included in this category are such ancillaryequipments as electric motors and generators. Thistype of interference is grouped into two categories:narrowband and broadband.

(1) Narrowband interference. Narrowbandinterference sources consist of cochannel, adjacent-channel, harmonic, and other signals at distinctfrequencies as described below:

(a) Cochannel and adjacent-channel operation.A transmission from a distant station, although fullyauthorized, may become a source of interference if it isunintentionally received along with a desired signal.This situation is usually prevented by assigningcochannel frequencies to equipments only withadequate geographical separation. Adjacent channeloperation may require some geographical separationalso; however, receiver selectivity usually providesrejection of undesired adjacent channel signals.Conditions which cause cochannel and adjacent channelinterference, then, include unusual propagationconditions (which cause distant transmitters to appear tobe local RF sources); frequency spectrum crowding(which may result in adjacent channels being occupiedby transmitters at power levels that override receiverselectivity); use of excessively high, unauthorized powerlevels; and lack of proper maintenance procedures(which can result in off-frequency

operation through transmitter frequency drift and lack ofselectivity at the receivers). Note that, for the purposeof this discussion, a channel is defined as an assignedfrequency range of a given portion of the RF spectrumand may include from 1 to 2700 audio channels.

(b ) Harmonics of the transmitted frequency andother spurious emissions. Harmonics of the transmittedfrequency and other nonharmonic spurious emissionsare always present to some degree in the radiatedoutput of a transmitter. Well-designed transmitters,maintained in good operating condition and properlyloaded into antennas, will radiate only very low-levelspurious emissions. Improper tuning, loading, andmaintenance, and faulty transmitter and antennaelements can cause normally low-level spuriousemissions to increase to extremely high levels. Thesesignals are primarily generated in the frequencymultiplier or RF power amplifier stages of thetransmitter. In addition, harmonics and othernonharmonic spurious emissions of the transmitter localoscillator (LO) may be radiated. An example ispresented in figure 2-2, which shows the transmitterspurious emission measurements on an AN/GRT-22transmitter. Frequently, interference is caused byspurious emissions from a transmitter, rather than froma transmitter operating on the desired frequency.Frequency allocation plans usually prohibit twotransmitters operating on the same frequency if theantenna patterns and service areas are such thatinterference could be caused.

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TM 11-490-5

Figure 2-2. Transmitter spurious emissions for an AN/GRT-22.

(c) Intermodulation and cross modulation.Intermodulation, the combining of RF signals in areceiver front end or transmitter final stage, and crossmodultion, the superimposing of the modulation of anundesired signal on a desired signal, occur as a result ofcircuit nonlinearities. A nonlinear circuit is one whichdoes not pass or amplify all voltage levels or voltagepolarities equally. Circuit nonlinearities may occur fromsuch simple causes as corroded or loose antenna

connections and waveguide hardware, foreign materialinside waveguides, electron tubes whose operatingcharacteristics have changed through use, and changesin circuit resistance or capacitance values as a result ofoverheating or aging, all of which illustrates theimportance of preventive and corrective maintenanceprocedures. An overdriven amplifier becomes anonlinear device which clips or

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TM 11-490-5distorts an applied signal. An example of nonlinearoperation of a transistor is shown in figure 2-3. Thisfigure shows the manner in which the output voltagewaveform becomes distorted as a result of operationover the nonlinear portion of a transistor's base currentversus collector current operating curve. This distortedoutput contains frequencies which were not present inthe sine wave input signal. These are actualfrequencies which comprise the distorted output signalin that they respond to such actions as filtering,coupling, and frequency selection as though they hadbeen input to the circuit. In the case of two or more RFsignals being presented to an RF amplifier under suchnonlinear conditions, sum and differenceintermodulation product frequencies can be produced.Many nonlinear devices are used to perform usefuloperations, such as within a mixer stage where thenonlinearity is used to heterodyne or beat the incomingsignal frequency with the local oscillator frequency toproduce an intermediate frequency. A diode is anonlinear device which may be placed in a circuit so asto pass only the positive going voltage peaks, whileshunting the negative going peaks to ground. However,when the RF amplifier of a receiver is nonlinear, or isoperated over a nonlinear portion of its characteristiccurve (overdriven), it has the potential for producingcross modulation and interlmodulation products. Eitherof these types of distortion is objectionable.

Figure 2-3. Example of distortion as a result ofoverdriving an amplifier stage.

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1. Cross modulation, in contrast withintermodulation, does not involve the mixing of two RFsignals to result in intermodulation products at differentfrequencies, although it does occur as a result of circuitnonlinearities. If a nonlinearity exists, such as anoverdriven RF amplifier stage, an amplitude modulatedadjacent channel signal will cause the gain of theamplifier to vary with the modulation. In effect, thismodulation is transferred to the desired signal as itprogresses through the amplifier. The possibility ofexperiencing cross modulation problems as well asintermodulation problems can be decreased by followinggood maintenance practices including proper receiveralignment to maintain designed selectivity and linearitycharacteristics, maintaining good electrical connections,and replacing components as needed to maintainoptimum operational performance.

2. Intermodulation frequencies may beproduced at a receiver when two signals aresimultaneously received which, although they may beoutside the RF bandwidth of the preselector stage,override the preselection and are presented to the RFamplifier (fig 2-4). If a nonlinearity exists within the RFamplifier which allows the mixing of these two signals,intermodulation frequencies will be produced accordingto the following equation:

fI= mf1± nf2

wherefI= intermodulation frequency

f1 = frequency of interference closest to thereceiver tuned frequency

f2 = frequency of interference farthest from thereceiver tuned frequency

m and n = integers (1,2,3 and so forth) Anyfrequencies produced which fall within the RF bandwidthof the filter following the RF amplifier will be presentedas false desired signals to the mixer stage. Theresulting demodulated signals will appear at the audiooutput. As an example of the production of anintermodulation frequency, consider the followingexample from experiments on the AN/GRR-23 receiver:In this experiment, a squelch break occurred with nodesired signal present when two signals at 139.8 and140.2 MHz were transmitted in the vicinity of the victimreceiver. The intermodulation frequency responsible forthis squelch break was found to be 139.0 MHz, thetuned frequency of the receiver. Although bothfrequencies, 139.8 and 140.2 MHz, were outside the RFbandwidth of the receiver, their combined effect was tocause a squelch break to occur. Analysis revealed thatthe intermodulation frequency responsible was of the 5thorder, calculated as follows:

fI = 3fI - 2f2= 3(139.8) - 2(140.2) MHz= 419.4 - 280.4 MHz= 139.0 MHz

Therefore, fI = f0 = 139.0 MHz.

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Figure 2-4. Intermodulation products occurring at a receiver.

3. Intermodulation products may also occur in alink transmitter as a result of the mixing of signals fromtwo interfering transmitters in the final stage of the linktransmitter. Intermodulation products are generated inthe final stage according to the' same equationpresented for receiver intermodulation products. Thetransmitted signal consists not only of the desired signal,properly modulated, but the intermodulation productsalso at the desired signal frequency (or, at least, withinthe RF bandwidth of the link receiver). Theintermodulation product signals will contain vestiges ofthe interfering signal modulations which will be heard asnoise at the link receiver. An example of the

production and transmission of 7th order intermodulationproducts which could interfere with a 50-MHz link isshown in figure 2-5. Interfering signals entering the linktransmitter antenna are mixed in the final stage andretransmitted as 7th order intermodulation productsignals according to the following relationship:

fI = 4fa - 3fb= 4(51.2) - 3(51.6) MHz= 204.8 - 154.8 MHz= 50.0 MHz

Therefore, fI = f0 = 50.0 MHz.

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TM 11-490-5

Figure 2-5. Intermodulation products occurring at a transmitter.

4. Coupling of transmitter signals which resultin cross modulation and intermodulation may occurthrough radiation from one transmitting antenna toanother and conduction by means of the normaltransmission line; through conduction from onetransmitter to another due to improper grounding orbonding; or through radiation from a transmitter antennaor improperly shielded, bonded, or grounded transmitterto another transmitter which is also improperly shielded.Frequently, it has been found that poor or rustingconnections on guy wires for antenna supports or powerlines can cause serious interference problems. As hasbeen shown, intermodulation products may be producedat many sum and difference frequencies when thefundamental frequencies of two transmitters are presentin the same nonlinear element. Interference may alsooccur when three signals mix in a nonlinear element.The sum frequency of two of the signals may produce adifference frequency with a third signal. Of importanceare the intermodulation products which fall in the sameor adjacent channel of the band in which the desired linkequipment is operating. These are the productsresulting from such combinations as:

2fa - fb, 2fb - fa, 3fa - 2fb, and fa + fb - fc.Higher order frequency intermodulation products areusually not detected, even though they are present,because they are of a very low power level.

(2) Broadband interference. Broadbandinterference is characterized by noise emission whichextends over a wide frequency range. One means bywhich wideband interference is produced is the periodicopening and closing of a current-carrying circuit, whichcauses sudden voltage changes (signal transientscontaining steep wavefronts). These transients arecomposed of many frequencies that are even multiples(harmonically related) of the occurrence rate of closingand opening the circuit. Undesirable responses appearin a receiver throughout a broad tuning range whensignals at these frequencies are radiated and detected.Interference sources of this type are discussed in (a)through (h ) below.

(a) Powerline. Powerline interference is a resultof any sudden change of voltage which may accompanysurges, corona, sparking, or the interruption of anelectric power circuit. Typical sources of powerlineinterference are: insulators

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covered by dust or other material deposited from the air.damaged insulators, excessive voltage, loose tie wiresor connectors to the powerlines, insulation breakdown intransformers or capacitor banks, intermittent contactbetween any two metallic objects on power poles, poorground connections between a neutral line and earth,and any sharp points projecting from powerlines.

(b ) Fluorescent lights. Transients may begenerated by the normal ionization of fluorescent lightswithin the tube or by defective starter and ballast units.The interference from fluorescent lights may arrive at areceiver by direct radiation of the steep wavefronts fromthe tube to the receiver antenna, by conduction from thelight fixture through the powerline, or by radiation orconduction from the tube to the powerline andreradiation to a receiver antenna.

(c) Arc welding equipment. Interference may becaused by the transients associated with a welding arc.The interference may be suppressed by propershielding, bonding, bypassing, and grounding. Weldingequipment in general should be located in a positionpermitting the shortest possible grounding connection toearth. High capacity, reactance-type welders of the typeused in industry are usually equipped with adequatesuppression components. However, lack of propermaintenance of these units may result in interferenceradiation over a broad frequency range for longdistances. When units of this type are found to becreating interference, they should be inspected and anyloose connections, broken bonding straps, defectivefilters, or defective bypass capacitors should be repairedor replaced.

(d) Ignition noise. The interference caused bytransients in vehicular ignition systems is usually mosttroublesome in the lower portion of the VHF frequencyrange (30-150 MHz). It is, however, detectable atfrequencies of 0-500 MHz. Instances have beenrecorded of interference being detectable and in somecases troublesome at distances greater than 500 feetfrom its source. Arcing produced in the low-tension sideof the ignition system by the contact-breaker points andsparking in the high tension system by the spark plugs isusually responsible. Additional sparking may occur ifthe ignition harness is defective. Steep wavefronts,associated with the electromagnetic fields produced bythe arcing and sparking processes, contain manypotential interference frequencies which may radiatedirectly from the discharge, from lowor high tensionwiring, or from any metallic object not properly bondedor grounded.

(e ) Switches and relays. All switches causeessentially the same types of interference, whether

electrical, thermostatic, or electromechanical inoperation. A switch is a device that can abruptly changeits electrical impedance from zero to infinity or frominfinity to zero. Transients throughout the circuit. whichresult in generation of broadband emissions, may causeinterference. A relay is a switch whose contacts areopened and closed mechanically by an actuatorcontrolled by the electrical field of a coil when anelectrical signal is applied to the coil(electromechanically actuated). In the case of manuallyactuated switches, the emission produced is generally ofrelatively short duration unless there are capacitors orinductors in the circuit. Usually such emission is eithernonrepetitive or is repetitive at a slow or irregular rate.Switching interference is more severe when largecurrent values or highly inductive circuits are involvedbecause electrical discharge across switch contacts(without capacitor protection) when making or breakingcircuits greatly intensifies the interference in both leveland duration. Electromechanically actuated switchessuch as relays, vibrators, and buzzers create exactly thesame type of broadband emissions as do manuallyactuated switches, but usually at a faster rate.Repetitive interrupters, such as vibrators, produce thesame broad spectrum emission on a continual basis.The steep wavefronts thus generated are possiblesources of interference. The signals present in thesesteep wavefronts may be radiated directly to a receiverantenna or they may be induced into the vibratorsolenoid and conducted through power and control lines.

(f ) Rotating machinery. Interference isgenerated in rotating machinery (e.g., motors orgenerators) due to the transients from brush bounce,commutator surface irregularities, brush arcing, andstatic discharge. Any rotating machinery with slidingcontacts may be regarded as a potential source ofelectrical interference. A motor with clean commutatorcontacts generates less interference than one withrough, dirty contacts. The switching and arcing processof commutation causes rapid current and voltagechanges which distribute interference energy throughouta wide frequency range. Another source of interferencemay be static discharges between a rotating shaft andbearing surface brought on by the insulation or dielectriceffect of the lubricating film.

(g ) Radar pulse. The radar output signal (pulsetrain), generated as it is by very rapid voltage changesand radiated as periodically occurring bursts ofelectromagnetic energy, contains frequencies whichspread over a wide band. These frequencies consist ofa wide range of harmonics of the basic frequency ofoccurrence of these pulses, which is the pulse-repetitionfrequency, as well as at the harmonics

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of the radar's carrier frequency. If the radar antennaradiating this train of pulses sweeps in a circular arc, theintensity of the pulses will seem to vary in intensity asheard at a fixed radio station. Radar pulse interferencemay be radiated from any metallic portion of a radarsystem structure as a result of coupling from the circuitcomponents which convey the pulses.

(h) Industrial, scientific, commercial, and medicalapparatus. Any apparatus using RF energy can be aserious source of interference. Such apparatus may bediathermy equipments, X-ray machines, microwaveovens, or induction heaters. Electromagneticinterference is radiated directly from the apparatus orconducted along the power and control lines. This typeof interference may travel hundreds of miles and oftencannot be detected in the area between the source andthe receiver being disturbed.

2-3. Transfer Mediaa. General. If electromagnetic energy is to

cause interference, it must be transferred from the pointof generation to the location of the susceptible device.This transfer may occur over one or more paths bymeans of conduction, radiation, or combinations ofthese two modes of transfer.

b. Conduction. Interference by conductionoccurs when signals are transferred unintentionallybetween two circuits by an actual circuit element whichis shared by the two circuits (common impedance), or byan effective common impedance (although no actualshared circuit element is present), caused by theproximity of circuit wiring. A complete circuit must exist;that is, there must be a signal path and a return pathbetween the affected circuits. Sometimes these pathsand effective common impedances are not readilyapparent, but they can occur, for instance, by directwiring or use of a common ground. Coupling ofundesired signals by means of common connections,especially on the ground side, is a frequent occurrence.Further description of interference coupling byconduction is contained in (1) through (3) below (seealso para 2-4, Equipment Susceptibility):

(1) Two circuits are said to be mutually coupledwhenever voltages or currents in one circuit inducecorresponding voltages or currents in the other circuit.The sharing of a wire or a junction point between two ormore circuits (common interconnection) can result incommon impedance coupling, whereby current in onecircuit causes a voltage to appear in another circuit.The interference voltage level so produced is dependentupon the magnitude of the common impedance and thecurrent flow through the impedance. Circuitscharacterized by high impedance (and low power levels)

are highly susceptible to interference, especially fromhigher power circuits that share a common impedance.The so-called ground loop, where undesired signals aretransferred from a highpower circuit to a low-powercircuit as a result of the impedance shared in a commonground circuit resulting from a poorly designed groundsystem or poor ground connection, is one example ofthis.

(2) Two conductors close together tend toproduce voltage variations within their circuits throughmutual capacitance. The capacitive reactance betweentwo conductors varies with the distance between theconductors, conductor sizes, and the frequency of thesignal being coupled. The effect of capacitive couplingincreases with increasing frequency. The use of ashield (or shielded wires in the case of conductors inproximity) will reduce the interference.

(3) The various circuit interconnectingequipments at a communications site cause closedloops to exist with mutual inductance acting as themechanism for interaction between the loops. Thisinteraction may be thought of as a transformer actionbetween the interference source and the sensitivecircuit. Circuits characterized by low impedances areparticularly susceptible to inductive coupling. The effectincreases with increasing frequency and decreasingdistance separation. The use of a magnetic shield orthe repositioning of equipments or wiring can reduceinductive coupling.

c. Radiation. Electromagnetic radiation of anelectrical signal involves propagation through a medium;therefore, the effects on the signal by the medium mustbe considered. During transmission of an interferingsignal, losses are incurred which can render the signalharmless. If not enough loss is involved, however, theundesired signal will reach a receiver as interference.When signals are radiated through the medium of theearth's atmosphere, the losses incurred in transmissionare dependent upon frequency, separation distance (thedistance from the emitter of the signal to the receiver),terrain over which propagation occurs, and theelectromagnetic properties of the transmission medium(which change with day and night, seasons, weather,and ionization effects of the sun).

(1) Table 2-1 shows the propagationcharacteristics of the RF spectrum. The three types ofradio wave propagation represented in this table are, inthe order of increasing frequency, ground waves(propagated over the surface of the earth and influencedby the ground as well as the troposphere, theatmospheric region immediately above the earth),skywaves (propagated by refraction effects of theionosphere, the region above the troposphere), .andtroposphere scatter waves (propagated by

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reflections at abrupt changes in the troposphere). Alongthe surface of the earth, the maximum distance that aground wave is effective decreases with increasingfrequency of the wave. Ground waves undergodeviation from normal, straight-line travel by contactwith the troposphere, refraction, reflection from pathdiscontinuities, diffraction, and propagation along thecurved surface of the earth because of the earth'sconductivity. It is for these reasons that ground wavepropagation extends

beyond the line-of-sight distance. Skywaves andtropospheric scatter waves undergo deviation indirection along their propagation paths as a result ofdiffraction, reflection, and refraction as they encountervariations in the media through which they travel. All ofthe above factors, in addition to absorption of the energyof the radio waves, are responsible for propagation pathlosses encountered by radiated signals and the"freakish" interference which results from distant, low-power transmitters.

Table 2-1. Propagation Characteristics of the RF SpectrumFrequency Band Propagation characteristics Typical usesBelow3 kHz ELF Primarily ground waves; low attenuation, reliable, Very long distance point-to-point (greater

daytime absorption of skywaves greater than at than 1000 nautical miles)night

3-30kHz

VLF Same as ELF, except attenuation equally low, day or night; reliable

Very long distance point-to-point. Tactical broadcast communications

30-300 LF Same as ELF Long and medium range (50 to 1000kHz nautical miles point-to-point com-

munications, marine, navaids300-3000 MF Ground waves, but also some ionospheric Broadcasting, marine communications,kHz skywaves; attenuation of skywaves low at night navaids, harbor telephone, medium and

and high in daytime. Subject to ground-skywave interference for distances less than 500 nautical miles.

short range

3-30 VHF Primarily skywave transmission over great Moderate and long distance communicationsMHz distances, depending on ionosphere. Varies, of all types

greatly with time of day, season, frequency, and portion of solar sunspot activity cycle. Subject to ground-skywave interference at short distances

30-300 VHF Primarily line-of-sight transmission. Sporadic Short distance, line-of-sight com-MHz ionosphere effects occur during high portions of munications, television, FM broadcasting,

solar sunspot cycle navaids, radar, troposcatter com-munications, aero-navaids

300-3000 UHF Substantially straight-line propagation analogous Short-distance communications, radar,MHz to that of light waves. Unaffected by

ionospheretelevision, aero-navaids, point-to-point relays, troposcatter communications

3-30 SHF Same as UHF Short-distance communications, radar,GHz point-to-point relay systems, navaids,

satellite relays, tropospheric scattercommunications

BandsELF =Extremely low frequencyVLF =Very-low frequencyLF =Low frequencyMF = Medium frequencyHF= High frequencyVHF= Very-high frequencyUHF= Ultra-high frequencySHF= Super-high frequency

(2) The ionosphere is a region of the earth'supper atmosphere which lies between about 40 to 50 kmabove the earth out to about 12,000 km above the earth.There are within this space three ionized layers or"regions," referred to as the D, E, and F regions,occurring at approximate heights of 50-90 km, 90-130km, and above 130 km, respectively.

These regions are believed to be generated byultraviolet light and "soft" X-rays from the sun. Radiowaves propagate through, or are reflected and refractedby, these regions, depending on the frequency of thewaves and the angle of incidence of the wave frontrelated to the layers. The regions are not uniformaround the earth, but, since they are

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affected by radiation from the sun, some portions of theregions may be more heavily ionized than otherportions. This can disrupt long-distance HF radiocommunications.

2-4. Equipment Susceptibilitya. General. Any equipment or device capable

of responding to electromagnetic fields or to electricalsignals must be considered susceptible toelectromagnetic emissions. Whereas paragraphs 2-2and 2-3 described the generation and transmission ofinterference signals, this section deals with the receptionof, and reaction to, these signals by various equipments.Two broad categories of equipment can be established:those that are frequency selective and those which arenot frequency selective.

b. Entry Mechanism. In paragraph 2-3 it wasstated that undesired signals are transmitted byconduction or radiation, or both. There are three meansby which an undesired signal may enter equipment: (1)through a common interconnection, (2) throughcapacitive and inductive coupling, and (3) through directimpingement of electromagnetic energy. Figure 2-6depicts examples of these entry mechanisms. In figure2-6A, undesired signal current is conducted from itssource in equipment A to the receiving unit, equipmentB, through common input leads. The use of commonpower supplies not properly isolated electrically(decoupled), or improperly operating multiplexingequipment are examples where this type of entry ispossible. Another means of entry through the commoninterconnection is shown in figure 2-6B. Even thoughthe common connection point is ground, it must beremembered that some impedance may be presentwhich is shared by the two signal paths and, therefore,coupling between the circuits takes place. This type ofcoupling is commonly encountered as a result of poorequipment design, installation, or maintenance, andmay be minimized by correct design of the groundsystem. Figure 2-6C shows an example of the entry ofundesired signals through capacitive and inductivecoupling between signal lines placed close together.This type of coupling can occur between multiple signaland control lines, other signal lines, adjacent powerlines, and transmission lines. Direct impingement ofelectromagnetic energy as a means of undesired signalentry is shown in figure 2-6D. Electromagnetic energycan also gain direct entry into equipments throughventilation holes, meter and other panel openings, andinadequate shielding, or indirect entry by causingcurrents to be induced in equipment cabling, connectors,and other wiring. The latter cases usually originatewithin a system or equipment and may be regarded asintrasystem coupling.

Figure 2-6. Interference signal entry paths.

c. Equipment Susceptibility Characteristics.(1) Frequency Selective Equipment.

Frequency selective equipment includes all equipmenttypes which operate at fixed frequencies or which aretunable over a selected range of frequencies.Communications receivers are subject to morenumerous, more complex interference situations thanother equipment types because of their highly sensitivefront end circuits and the fact that so many users mustshare the RF bands. A discussion of typical interferencesituations follows:

(a) Cochannel and adjacent channelinterference. Any transmission may interfere withanother system if it is present simultaneously with adesired RF signal. This interference can occur whentwo transmitters have been assigned to operate on

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TM 11-490-5the same frequency or channel or when the transmittershave been assigned adjacent channels. Interferenceoccurs when the carrier frequency or sidebandfrequency of the interferer is within the bandpass of avictim receiver (fig 2-7 and 2-8).

Figure 2-7. Cochannel interference.

Figure 2-8. Adjacent-channel interference.

(b ) Intermediate-frequency interference. A formof interference known as intermediate frequency (IF)interference may occur if a high-amplitude signal at thereceiver IF reaches the mixer or IF stage (fig 2-9)through the receiver antenna and front end, or throughthe equipment case or cabling. Since the IF of areceiver normally remains constant through the tuningrange of the receiver, this type of interference ischaracterized by its presence at all tuned frequencies ofthe receiver.

Figure 2-9. Intermediate-frequency interference.2-13

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(c) Spurious response interference. In additionto the susceptibility of receivers to cochannel. adjacentchannel, and IF interference, most receivers may alsobe susceptible to interference at other frequenciesbecause of their "spurious" response characteristics; thatis, their response to frequencies other than the tunedfrequency of the receiver. Most spurious responsecharacteristics are the result of nonlinear elements ofthe RF circuitry (front end) as well as in the mixer stage.which is designed to be nonlinear. Although a signalmay be relatively free of harmonic content, harmonicsof the signal may be generated in the receiver front endwhenever a nonlinearity is encountered. Such nonlinearelements include mixer stages and, frequently, antennagrounds or other signal connections (oxide films formedon the metal by corrosion), cold solder joints, andoverdriven RF stages. An input signal will mix with localoscillator harmonics. harmonics of an input signal willmix with the local oscillator signal, or combinations ofboth may occur. If any combination of harmonicsproduces a signal at the same frequency as the IF, atthe mixer stage output, interference may occur. Mostspurious responses (fsp ) can be identified by thefollowing relationships:

For a single-conversion receiver:

For a dual-conversion receiver:

Where p is an integer or zero denoting the harmonicorder of the local oscillator: q is an integer denoting theharmonic order of the spurious response frequency; fuand fIF denote the local oscillator and intermediatefrequencies, respectively. the numerical subscriptsindicate the number of conversions preceding thesection of the receiver of concern, and the positive andnegative signs identify different spurious responses. Asan example of spurious response identification, considerthe AN/FRC109(V) tuned to 7680 MHz. The localoscillator frequency is 7750 MHz and the intermediatefrequency is 70 MHz.

For p = 2, q = 2 and sign of(— ):

In other words, the second harmonic of the spurioussignal frequency mixes with the second harmonic of thelocal oscillator frequency in the mixer stage to produce apotential interfering signal at 70 MHz which is identicalto the IF of the receiver. The IF amplifier stages willtreat this signal in the same manner as the desiredsignal.

(d ) Communications receivers are susceptibleto intermodulation and cross modulation interferenceresulting from simultaneous reception of two or moresignals combined through circuit nonlinearities in thereceiver front end. The result of this combining (mixing)may be the production of sum and differencefrequencies within the receiver pass band which areconverted to the intermediate frequency in the mixer,demodulated, amplified, and presented as noise in theaudio output. The interfering signals do not have to bewithin the receiver's preselection pass band for this tooccur. They must merely be of such sufficient strengthas to override the selectivity capabilities of the receiver.Cross modulation may be apparent to the operator as anunclear signal, or as the receiving of two signals ofdifferent intensity.

1. Interference due to intermodulation mayoccur at a receiver when the frequencies of two signalsare separated by an amount equal to the frequency towhich the receiver is tuned. If two such signals arepassed through a nonlinear element in the receiver frontend, their sum and difference frequencies are generatedby heterodyne action. These new frequencies aretermed the A-B and A + B intermodulation products.When these frequencies are of sufficient amplitude andare within the receiver pass band, they will be detectedin the same way as a desired signal.

2. Interference due to intermodulation may alsoresult when the harmonic of one signal and thefundamental of any other signal combine to produce asum or difference frequency equal either to the receivertuned frequency or to a spurious-response frequency.Any multiple of the tuned frequency, such as 3A±2B,3B±2A, 4A±3B, and so on, may cause interference ofthis type if it is of sufficient amplitude.

3. Interference due to intermodulation may alsooccur when three signals combine to generate thefrequency to which a receiver is tuned. Two signalfrequencies, when combined with a third frequency in anonlinear element, will produce sum and differencefrequencies. If these frequencies are equal to either thetuned frequency or a spurious-response frequency of areceiver, detection will take place and interference willoccur. This frequency combination is termed theA±B±C intermodulation products.

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(2) Non-frequency selective equipment. Thisterm refers to all electrical and electronic devides otherthan those which are tunable to, or fixed at, a selectedband of frequencies. In general, because of thecomplexity of types, arrangements, and operationalcharacteristics of equipments, and because of the manytypes of interference-causing signals, entry modes, andinteractions, the susceptibility characteristics of thisequipment category have not been completelydocumented or standardized. Susceptibility of individualequipments used for specific requirements is bestdetermined by test, either by simulation of interferencesignals or under actual operating conditions. A briefdiscussion of some susceptibility characteristics follows:

(a ) Digital computers. Since operation of digitalcomputers depends upon pulses and levels of fixedamplitudes occurring at predetermined times, computersare particularly susceptible to interference from pulsedsignals (such as from radars sweeping the area).Experimental investigations of problems resulting frominterference have demonstrated false switching in flip-flops (circuits designed to produce one of two possibleoutputs, depending on the last input signal received) andlogic gates (circuits whose output depends on two ormore input signals), deteriorated signals, and erroneousoutputs. Computers are also susceptible to another typeof interference which is worthy of mention, even thoughit is not electromagnetic. Computer memory banks aresusceptible to magnetic fields and if these fields are ofsufficient strength, data stored in the computer memorymay be lost. Therefore, strong magnetic fields fromeither permanent magnets or electromagnets should notbe allowed in the vicinity of computers.

(b ) Control devices. This equipment categoryincludes amplifiers which have low-level, high-impedance inputs and are used in the processing ofsensor signals, control of electromechanical devices,and many other functions. Examples are

servoamplifiers and dc amplifiers. Since this type ofequipment frequently operates by controlling theamplitude of signals at the 60-Hz and 400-Hz powerlinefrequencies, they are particularly susceptible topowerline transients and stray coupling to wiring andtransformers. Because they operate at relatively lowsignal levels, amplifiers of this type are troubled bycommon-mode coupling at their input terminals.Rectification (removal of positive or negative-goingvoltage peaks) of high-frequency signals by nonlinearcircuit elements can cause saturation (operation at themaximum limit of response regardless of input voltagevariations) or desensitization (lack of response to low-level signals) of gain characteristics, or parasitic(unintended, self-sustaining) oscillations.

(c) Displays. Cathode-ray tube displays aresusceptible to emissions from radars, communicationtransmissions, ignition systems, and other equipments.The main effect is the disruption of the visualinformation presented to the equipment operator.Cathode-ray tubes are susceptible to stray magneticfields which deflect the electron beam. Interference insweep circuits is characterized by distortion of thedisplay (by causing the sweep to be nonlinear), whileundesired responses in the video sections may appearas intensity modulations. The circuits which process thevideo, or information signal, and those that generate therequired sweep voltages are subject to powerline andcommon-mode coupling, as outlined in the previoussection.

(d) Test equipment. Test equipment is just assusceptible to unwanted electromagnetic energy asmany of the equipments under test. Powerlinetransients and capacitive and inductive coupling mayproduce false indications when testing equipment.Noise may be introduced into the test equipment as aresult of a grounding problem and thus increase theindicated noise level.

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CHAPTER 3

EMC PROBLEMS AS RELATED TO SIGNAL

TYPES AND MULTIPLEXING TECHNIQUES

3-1. GeneralThe manner in which the interference manifests itself inthe system output will vary with the multiplexingtechnique and with the signal type being transferredover the system. These differences are important in theisolation and identification of EMC problems.

3-2. Signal Types and Multiplexing Techniquesa. General. Currently two basic types of signals

are transferred over communications systems. One

type, generally referred to as analog, uses the actualvoice or other continuously variable waveform tomodulate the transmitter directly. The other type,referred to as digital, transfers an original digital datastream or converts the input analog waveform to adigital bit stream of "ones" or "zeros" and uses thisdigital bit stream to modulate the transmitter. Typicalanalog and digital signals are shown in figure 3-1.

Figure 3-1. Typical analog and digital signals.3-1

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b. Analog Signals.(1) In an analog system. a continuously

variable waveform is transferred. Any interferenceentering the system is added to that waveform. resultingin a composite of both the desired analog signal and theinterfering signal: this is shown in figure 3-2. As thelevel of the interfering signal increases. the amount ofdegradation in system performance encountered can .begradual and the overall system performance will dependupon the types of interfering signals. the interference

mechanism, and the relative levels of the desired andinterfering signals.

(2) Since both the desired and interferingsignals are present in the system output signal, theoutput signal can be monitored with an oscilloscope orother appropriate display device to indicate thepresence and type of interference being experienced.'With proper equipment and experience, an operator canusually identify the type of interference (noise, digital.radar, jammer, etc.) and thus determine the nature ofthe problem.

Figure 3-2. Example of analog signal with random noise interference.

c. Digital Signals.(1) Digital communications systems do not

transmit an actual analog input signal. Instead, theinitial signal is transmitted digitally. For example, theanalog signal may be sampled at some specific rate andeach sample converted to a digital signal representingthe amplitude of the analog signal at the time it wassampled. Figure 3-3 shows a binary digital signalconsisting of "ones" and "zeros." This digital signal istransmitted to the receiver where it

is detected and converted back to the original analogform. Interference is not additive as in the analog case,but changes the relationship of the "ones" and "zeros"for the particular sample; this is shown in figure 3-4.Therefore, the interference does not appear directly inthe output signal but causes a change in the amplitudeor some other characteristics of the reconstructedanalog signal for the particular sample or samples.

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Figure 3-3. Conversion of analog to digital signals.

(2) Since interference in a digital system causeserrors in the digital bit stream, some method of errordetection can be used to monitor system 'performance.If adequate error correction cannot be provided,retransmission must be requested, either automaticallyor manually. In a voice channel these errors can causea change in one or more words with little indication thatinterference is present. In a data channel, erroneousdata is the result.

(3) Most digital systems require some type of timesynchronization between the encoder at the transmittingend and the decoder at the receiving end. Thesusceptibility of this synchronization

system to interference is a major factor in the error ratethe system can tolerate without total loss of informationtransfer.

(4) Since the interference does not appear directlyin the audio channel output, the output will not providereliable information for interference identification.However, the total or composite signal is generallypresent in some form at the receiver detector.Therefore, by monitoring this composite signal prior topulse detection and restoration. an appropriate displaycould be used to detect and identify interference in amanner similar to that used in an analog system.

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Figure 3-4. Example of digital .signal with interference.

d. Multiplexing Techniques. In order to transfermultiple audio channels over a single radio link, amethod of combining or multiplexing the individualchannels into one transmitted signal and separating orde-multiplexing the channels at the receiver must beused. The two most common techniques employed inmilitary communications are frequency divisionmultiplex (FDM) and time division multiplex (TDM) (fig3-5). While FDM is currently the prime technique formultiplexing, there is a trend toward increasing use ofTDM.

(1) Frequency division multiplex. In an FDM

system the audio channels are assigned to discretefrequency bands just wide enough for the audio signalbeing transferred. The total frequency bandwidth is thesum of the total number of frequency bands desired.This signal is then transferred over a single transmitter-receiver link. At the receiver, the overall signal isdetected and the discrete frequency bands areseparated for transfer over individual audio channels.Depending on the intensity and frequencycharacteristics of the interfering signal, only a fewchannels may be affected or the whole system may beadversely affected.

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Figure 3-5. Examples of channel distribution for FDM and TDM.

(2) Time division multiplex.(a ) In a TDM system each audio

channel is assigned a discrete time slot. Since TDMsystems are digital in form, the characteristics andsequencing of the time slots are determined by theanalog-to-digital conversion system employed and thenumber and nature of the audio channels beingmultiplexed. In the case of current tactical pulse codemodulation (PCM) systems, the sample rate for theanalog-to-digital conversion is 8,000 samples persecond. Since 6 bits are allocated to each sample, atotal of 48,000 bits per second (48 kb/s) are allocated foreach audio channel. Channel separation is related totime rather than frequency. Therefore, a method ofscynchronizing and maintaining the time relationship ofthe transmitted and received signal is required.

(b ) Because the channel separation is afunction of time, the information on each channel istransmitted on the total RF bandwidth of the transmitterinstead of on a discrete frequency. Therefore, anarrowband, continuous-wave interferer

would affect every audio channel in the same manner.A random pulse interferer would also interfere with allchannels equally. A pulse interferer operating at thesynchronizing frequency of the system couldtheoretically interfere with one channel only; however,this occurrence is improbable.

(3) Multiple multiplex. Both types ofmultiplexing systems can be double multiplexed. Thatis, a small group of audio channels can be multiplexedand then combined or remultiplexed with other smallgroups of audio channels into a large group ofmultiplexed channels for transmission over the RF link.This approach enables small groups of channels to beextracted from the larger group at an intermediatecommunications site in a multiple relay system for localdistribution, and other local channels reinserted in thelarger group without disturbing or demultiplexing thetotal number of channels being transferred. This doesnot change the interference mechanism itself, exceptthat it is theoretically possible to isolate and jam one ofthe small groups of an FDM system.

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TM 11-490-5

CHAPTER 4

DETECTION, RECOGNITION, ISOLATION, AND

REMEDY OF EMC PROBLEMS

4-1. Generala. EMC problems at fixed Defense

Communications System (DCS) installations or at fixedor transportable non-DCS facilities may be grouped intotwo broad categories:

(1) Instrasite EMC problems associated withthe equipments and installation which generally requiresome type of maintenance to resolve.

(2) Intersite EMC problems when theinterference originates at some location other than thecommunications site.

b. A DCS station is a vital link in a chain ofcommunications which may provide service betweenlocal subscribers or between far-distant points and localsubscribers, and may interface with non-DCS facilities.The station .may handle clear-text or encryptedtransmissions. The stations may be connected by radiopropagation media only, or in combination with cablecircuits. However, with the exception of unattendedrelays, stations are manned by operating personnel whoare vital in maintaining successful communicationswhen interference is experienced. At most stations,some incidence of EMC problems may be anticipated.These problems may originate at some location notassociated with the communications site, they may bethe result of enemy actions, or they may originate withina station. When new equipment is procured, extensiveEMC tests are made in an effort to eliminate EMCproblems. However, these tests do not alwaysreproduce all conditions in an actual installation. Duringacceptance tests of a new installation, it may not alwaysbe possible to make a complete test covering the entirefrequency range which is to be used in operation. Asthey age, components drift out of tolerance andsometimes fail completely; when this happens, rapidaction must be taken to isolate, locate, and repair orreplace the component. The human factor is thus ofvital importance in the solution of EMC problems.

c. To facilitate rapid solution of EMC problems,operation personnel must be familiar with all facets oftheir station. They must understand its scope andcapabilities, be familiar with normal indicator readings(as they usually provide the first indication of anyproblems), and be conversant with the use of availableequipment. Operating personnel must also learn torecognize situations which require more skilled

assistance and to report promptly any problem outsidetheir experience. The following list, although notexhaustive, contains some of the "tools" available tooperating personnel:

(1) Test equipment.(2) Testing and isolation procedures.(3) Equipment alignment procedures.(4) Backup or spare equipment and modules.(5) Patching and rerouting techniques.(6) Interpretation of equipment meters and

indicators.(7) Antenna orientation.(8) Communication training.(9) Substitution.

(10) Signal tracing.(12) Experience of senior operating personnel.(13) Equipment history (log books and the

like).

4-2. Discussion of EMC Problemsa. Introduction.

(1) When a communications site is installed,intrasite conducted emission problems have normallybeen reduced to acceptable levels by proper grounding,equipment location, and similar installation procedures.However, normal deterioration will occur over a periodof time, corrosion may form on grounds and otherterminals, connections may develop higher resistancesthan normal, and components may degenerate or failwithout causing an apparent equipment malfunction. Allof these conditions may cause interference. Theseinterference problems can normally be avoided orminimized by proper preventive maintenanceprocedures.

(2) One major cause of operational EMCproblems is radiated signals from sources beyond thelocal C-E site. This type of intersite interference isdifficult to isolate or minimize. It may be unintentionalinterference from friendly (or enemy) radio or radartransmitters, or it may be intentional jamming orintrusion.

(3) Although interference may originate froman RF source, it usually manifests itself as a faultyoperating condition in a subscriber's instrument. It

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TM 11-490-5

may first be reported by the subscriber, or be discoveredduring routine circuit checks.

b. DCS (Fixed Installation ).(1) In the case of a fixed DCS system, the

equipment is normally installed in accordance with I)('Astandards and other installation criteria. Any deviationfrom these standards is corrected or a waiver isrecorded when the system is accepted and placed inservice. Daily logs concerning equipment performanceare maintained and a history of any discrepancies andcorrective action taken are recorded. Therefore,operating personnal at a DCS fixed site have both thestandard performance criteria for the particularequipment and the past history of the performance ofthe facility. If operating personel are cognizant of thepast performance of the system, as recorded in thestation log, they can readily recognize any deviationfrom normal performance. The data also provideinformation on whether the problem is a recurring oneand what corrective action was previously taken. Thedata may also assist in the isolation of the problem anda determination of whether it is an EMC problem orequipment malfunction.

(2) Intrasite EMC problems can often betraced to a lack of adequate preventive maintenancewhich can result is partial or complete failure ofcomponents. either causing electromagneticinterference (EMI) or increasing EMI vulnerability. Theusual cause of loose connections. poor groundconnections, corroded terminals. cold solder joints, andfaulty cables is lack of proper maintenance. It is worthnoting that the installation of new equipment withaccompanying cables and jumpers can be the cause ofEMC problems not previously present. Existing cablesnot associated with the new installation can be relocatedor damaged to the degree where interference occurs.All new installations or replacement work should bechecked for their effects on other circuits.

(3) Intersite unintentional interference at aDCS fixed site is generally caused by installation oroperation of new equipment, new frequencyassignments, or weather conditions. This results in asite becoming more vulnerable because of physicalchanges at the site (such as grounding) or abnormalpropagation (such as ducting). Frequently, when thisoccurs, the interfering source may be identifiable fromfrequency assignments. However, if the interferingfrequency is assigned to the intefering station, and thelatter is operating within authorized bandwidth, mode,and emitted power limits, the solution will lie in obtaininga different frequency assignment for one station or theother. If this is not possible, steps must be taken tominimize the degree of coupling (for example, slightrotation of receive

antenna; use of a more highly selective receiver, ifavailable; or use of a frequency-synthesized receiverinstead of the automatic frequency control type,providing the distant wanted transmitter is alsofrequency-synthesized).

c. Non-DCS (Transportable).(1) In the case of non-DCS transportable or

mobile systems, each time the equipment is moved andreinstalled, new problems may be experienced.Although general performance criteria are available forboth equipments and systems, performance atindividual locations will vary. Equipment logs canprovide some insight into past performance of theequipment and are useful in establishing performancecriteria. However, the experience of the personnelassigned to the system as to normal performance undersimilar geographic, soil (ground considerations), anddistance conditions is the most significant factor in therecognition and isolation of an EMC problem.

(2) Intrasite EMC problems at a non-DCStransportable or mobile site are more prevalent than afixed site, because equipments are subject to movementand reinstallation, and therefore cables and connectorsexperience greater wear and breakage. Each newinstallation has a potential for problems with connectors,terminals, antennas, primary power, grounding, andinterconnections. The methodology for isolation andlocation of these types of EMC problems is similar to thefixed site, but the probability of occurrence is muchgreater.

(3) The source of intersite unintentionalinterference at a non-DCS transportable or mobile site isalso more difficult to isolate and locate than at a fixedsite. Since the system is the new element in the area.the number and location of potential interferers in thearea are unknown unless an extensive site survey andfrequency monitoring have first been performed.

d. Unintentional EMC Versus Intentional ECMProblems.

(1) When outside interference is detected,the operator must determine whether the problem isattributable to unintentional causes or whether jammingor intentional intrusion is occurring. Where the latter issuspected, any action taken must be such as to denythe enemy any intelligence regarding the effectivenessof his actions, or the fact that they have been detected.

(2) The procedures for dealing with enemyaction are prescribed by AR 105-3, "ReportingMeaconing, Intrusion, Jamming, and Interference (MIJI)of Electromagnetic Systems;" AR 105-87, "ElectronicWarfare;" and DCAC-310-70-6, "Jamming or Sabotageof DCS Telecommunications Facilities."

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4-3. Recognition, Isolation, and Remedy of EMCProblems

a. Introduction.(1) The most important factors in resolving

EMC problems are the knowledge operators have oftheir stations and the ability to speedily detect anyoperational abnormalities. A knowledge of normalmeter and indicator readings, signal "sounds" at audiofrequency, and visual representation of signals on acathode ray oscilloscope display or spectrum analyzerare all good tools in detecting abnormalities andeffecting rapid corrective action. This should becoupled with a thorough familiarity with all aspects ofstation configuration. The station log, together withperformance records for channels and systems, is aninvaluable record of problems and their causes. Use ofthese logs may facilitate and even expedite resolution ofEMC problems. A thorough knowledge of routinemaintenance checks will facilitate the development ofprocedures designed to remedy abnormalities causes byEMC problems.

(2) An interference problem can be isolatedto a particular element or equipment within acommunications site by signal tracing, if adequateequipment monitoring or test instruments are available,or by substitution when instrumentation is limited.However, in some cases, it can be extremely difficult todetermine precisely at what point (component,subsystem, or system) interference is being introduced.Frequently, resolution of a problem is achieved not by astudy of what is affected, but rather on determination ofthe source and the coupling medium, then controlling,reducing, or eliminating either the source or the couplingmedium. In any case, the important factor is to proceedmethodically rather than skip from equipment toequipment in a random manner. For instance, if troubleshows up on a channel, and it has been determined thatthe transmitting station is not at fault. the trouble couldlie within circuit modules at the channel level, such asline amplifiers; in channel, group, and supergroupdemultiplexer circuits: in baseband amplifier modules;within the receiver; or at the antenna. Probing of thesecircuits, in this order, at the system test jacks and use ofappropriate test equipment should isolate the problem.Substitution, in the event that test equipment is notavailable, involves the placing in service of alternatechannels, modules, and equipments by means ofpatching and special cabling, if necessary inemergencies. The substitution method, althougheffective in isolating interference to a particularequipment or its associated cabling, is generally slowand has the disadvantage of requiring interruption ofservice during substitution, unless parallel circuits canbe used to advantage without disturbing normal

operations. If available at the site, a battery poweredmultiband radio becomes an. effective diagnostic toolproviding aural indications which, when used inconjunction with an improvised directional antenna, canaid in determining the direction of the interferencesource. Aural indications can often be correlated toaural noise at a patch panel or output of equipment. If afrequency is noted, at which the type of interference isheard, the radio should be taken outside of the area oraway from the shielding influence of equipment so thatan attempt can be made to identify the source of EMI. Ifthe EMI is weaker outside, the radio should be broughtinside as the source is probably in the building. Animprovised loop antenna used with either the radio or afrequency selective voltmeter may be used to determinethe direction of the EMI source or to make a specificdetermination of the generating equipments and leadscarrying the noise. It may be necessary to obtain theassistance of maintenance personnel in fabrication of asuitable loop antenna. Being battery-powered, andtherefore free of the station's primary power source, theradio can be used to determine whether the interferenceis entering the system by RF radiation or through theprimary power lines. Poor grounds, cracked or dirtyinsulators, or defective transformers in the ac powersystem can, under certain conditions, be located bymoving the radio closer to the suspected interferencesource.

b. Recognition Procedure.(1) Determine origin of the interference.

(a)The effective clearance of asuspected EMC problem requires a rapid initialassessment of the origin of the interference. That is,was it generated within the local station, or was itgenerated by a distant station? In the latter case, thesource must be determined. To allow effectiveapplication of isolation procedures (para 4-3c ) thisdetermination must be made without delay.

(b) Although interference can beintroduced into a station from an external source bymeans of cables and transmission lines, the mostcommon avenue of entry in systems employing radioequipment is through antenna/receiver systems.Although interfering RF emissions may be locallygenerated, the majority are caused by distanttransmitting stations. Steps which can be taken todetermine whether the interference is entering thesystem by RF means are as follows:

1. Check whether the signal level hasincreased significantly at the receiver output. If it has,this may indicate an interfering signal on or close to thereceive frequency.

2. Display the output on a spectrum

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analyzer, if available, to see how it compares with anormal receive signal.

3. If necessary, have the distant desiredtransmission disabled, to allow a better visual and auralexamination of the receiver output to see whethercochannel interference is occurring.

4. If necessary, change the receiverantenna and/or alter the antenna azimuth slightly (ifpracticable) to observe any effects on the extent of theinterference.

5. If the above steps are inconclusive,disconnect the receiver antenna from the receiver andconnect in its place the carrier output of a suitable signalgenerator adjusted to the receive frequency, at anappropriate level. (If the receiver has a beat frequencyoscillator, it should be disabled.)

6. Apply a suitable degree of modulationto the signal generator and observe the receiver output.This is to ensure that the receiver is not introducing anyinterference due to nonlinearity.)

7. The next step is to arrange an RF"back to back" test, keeping the receiver isolated fromits antenna, as follows:

(a) Retain the connection from thesignal generator to the receiver input, but switch off themodulation.

(b) Connect the composite transmitoutput of a multiplex/submultiplex system to the"external modulation" -jacks or terminals of the signalgenerator. (Insure that the depth of modulation is suchas to provide linear operation.

(c) Provide keying on the channelswhich were suffering interference.

(d) Adjust the receiver output tonormal receive multiplex/submultiplex system levels. Inthis way a "back to back" arrangement is achievedwhich incorporates all receiving equipment except theantenna system. If the interference is still present, itoriginated within the station. If it is no longer present, itwas introduced by RF means. Remember, however,that it could be originating at the desired distant stationdue to a fault condition there.

(c) In making the above determination, it must beremembered that even if interference is introduced byRF means, it could be locally generated. However, thedetermination greatly facilitates isolation procedures.

(d) If the interference is entering the system by RFmeans, a check must be made to establish the source.Cochannel interference might require the distant wantedstation to disable its transmitter output to allow the localstation to demodulate and print (if possible) theinterfering signal as a means of identification. Adjacentchannel interference is more easily discernible and isusually easy to see on a spectrum analyzer or

oscilloscope. A portable radio or high impedanceearphones are also useful tools for an experiencedoperator to determine the presence and nature of RFinterference.

(e) If it is determined that cochannel RFinterference is present, the possibility of deliberateinterference or jamming by unfriendly forces must beinvestigated.

(f) If it is suspected that the interference isjamming, do no use the orderwire or any other form ofclean-text transmission to advise another station of theseverity of the interference, or even that interference ispresent, until it has been determined that jamming is notinvolved or the proper authority has been informed anda go-ahead has been obtained.

(g) If the interference is not being introducedthrough the RF medium, rapid isolation: , to eitherbaseband (audio) or the multiplex carrier will assist inrecognition of the type of interference. If theinterference was introduced into the baseband audiosignal or the multiplex system prior to transmission, itmay be difficult to recognize at the receiver unless theoperator has experienced this type of interferencepreviously. Therefore, the origin of the interference toeither the baseband audio or multiplex system cannormally be traced more rapidly by checking themultiplex system at both receiver and transmitter sites.Detailed procedures for isolating the entry point into thesystem are contained in (2) below; however, a check ofthe input and output signals of the multiplex equipmentscan provide a rapid determination of the multiplexperformance. If this performance is determined to benormal, then further isolation to cabling and switchingcircuits as detailed in c (3) (a ) and c (3) (b ) below maybe required.

(2) Determine nature of interference.(a) Monitor the channel or channels in

question with high impedance earphones or otheravailable test equipment to determine the nature of theinterference. Table 4-1 provides a listing of the audibleoutput to be expected from a receiver under variousinterference conditions and may be used to assist in therecognition of interference sources. Note that theconditions listed in this table will be in addition to thesignal normally heard, which itself may be in the form oftones, recognizable to experienced operating personnel,but from which it is impossible to extract the intelligenceof the signal by merely listening. That is, other endinstrumentation would be required for signals such asteletype, digital, and facsimile, in order that the mannerin which the intelligence of these signals is affected canbe determined. As an alternative, the desired signalcould be disconnected at the far station so that theinterference alone could be heard. Figure 4-1 shows

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TM 11-490-5the effect on an audio signal of some of the terfererslisted in table 4-1.

Table 4-1. Audio Output Responses Characteristic of Various Types of Interference

Receiver audible output Character of interference Possible source or mechanism

Reduced noise level (or steady tone Carrier (only) Cochannel, spurious, intermodulationwith beat frequency oscillator(BFO) operating)

Pulsed variation in noise level (or Pulsed continuous wave (CW) or Adjacent channel, co-channel,pulsed tone with BFO operating) digital transmission spurious, intermodulation, cross

modulationPulsed variation in noise level (two Unwanted radio teletypewriter Adjacent channel, co-channel,

pulsed tones with BFO operating) (RATT) frequency shift keying spurious, intermodulation, cross(FSK) transmission modulation

Added normal or distorted voice Unwanted voice transmission Adjacent channel, co-channel,spurious, intermodulation, crossmodulation

Whistling or squealing Unwanted transmission or in- Adjacent channel, co-channel,termediate frequency oscillation spurious, intermodulation, cross

modulation, parasitic and IFoscillation

Rapid variation in noise level (or Unwanted facsimile transmission Adjacent channel, co-channel,several pulsed tones with BFO spurious, intermodulation, crossoperating) modulation

Steady tone or whining High rate periodic pulses Radar, rotating machinesBuzzing Medium rate periodic pulses Buzzers, vibratorsPopping Low rate periodic pulses Ignition systems, magnetosHum Low (60 Hz) rate Power linesFrying High rate random pulses Electric arcs, continuously arcing

contactsSputtering High rate random pulses Arc welders, arc lamps, diathermyClicking Low rate random pulses Code machines, electric calculating

machines, mercury arc rectifiers,relays, switches, teletypewriters,thermostatic controls, electrictypewriters

Crackling Static or corona dischargesSharp crackle Ambient noise

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Figure 4-1. Audio signals subjected to various interference effects.

(b) The following 1. through 6. containinformation regarding the effects that various types ofinterference have upon communication systems.The purpose of this information is to assist in the

detection and recognition of the interference source.Table 4-1 contains audio output response characteristicsin a condensed form which can be used in recognitionprocedures. These specific recognition

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procedures are pertinent to both DCS and non-DCSfacilities.

1. Ignition noise. Privately owned vehicles inthe vicinity of a communications site normally do notcontain extensive noise suppression devices and aretherefore sources of ignition noise. The noise ischaracterized by a popping sound at a low periodic ratewhich varies with the speed of the engine. Gasoline-powered lawn mowers are also potential noise sourcesaround communications sites. Ignition noise from thesemachines results in high levels of radiated emissionsbecause of their relatively open construction andconsequent lack of shielding. The level of interferencewill vary, depending on the type of signal beingreceived. For example, since ignition noise has a ratherbroad RF spectrum, it will usually affect all channels ofan FDM system. In the case of PCM, which is timedependent, the number of channels affected woulddepend on those channels which were in the time slotwhen the interference pulses occurred. At low operatingspeeds, the interference pulses occurred. At lowoperating speeds, the interference may appear to berandom popping on a single channel. At high operatingspeeds, however, the PCM system may be totallyaffected.

2. Radar. A pulse-modulated radar can causebroadband interference similar to vehicular ignitionnoise; however, the periodic rate is directly related tothe PRF of the radar. If aural means (earphones) areused to monitor this type of interference, it will benoticeable as a definite and recognizable tone, asopposed to the popping characteristic of ignition noise.The intensity of the interference will also vary as theradar antenna rotates, if the radar is a scanning type.The visual indication will be that of a pulse movingacross the oscilloscope display (fig 4-1).

3. Office equipment. If offices are colocatedwith the communications site, office equipment,especially calculators and data processing equipment,present a potential interference problem. Theinterference will be manifested as a random rate clickingsound which affects most types of signal modulation.Improper filtering and poor grounding are commoncauses for this interference affecting communicationsequipment.

4. Power lines. Power lines are a potentialsource of many interference problems as a result ofinsulator and transformer breakdown, poor groundconnections, lightning discharge transients, andfeedback interference from equipment using a commonpower source. The interference may be in the form of a60-Hz hum, as in the case of poor ground connections,or poor earth grounding at the power pole; frying soundsas when transformer breakdown occurs. or insulatorsbecome damaged; or

high-pitched whine from rotating machines.5. Cables. Interference problems attributable to

cables may result from damage which has causedinsulation breakdown, or breaks in shields. Interferencemay be noted as hum which results from brokenshielding, improper shielding, or improper shieldconnections; or crosstalk as a result of signal-carryingpairs being in proximity, particularly when high signallevels are carried in proximity to low signal levels aswhen improper cabling, patching, or jumpering for agiven installation task occur.

6. Equipment grounding. Defective orimproperly installed earth grounds and equipmentgrounding systems are a major source of EMI problems.Primarily, the cause is an unwanted resistance commonto one or more circuits as a result of ground paths longerthan necessary or poor ground connections. Theseconditions are more frequently encountered in multipleground point systems rather than common pointsystems. The indications that grounding insufficienciesare the origin of EMI are varied and dependent upon thecircuit involved; however, the usual symptom ispowerline hum. In other instances, the problems maybe simply that equipment malfunctions occur because ofhigh impedance return paths which limit current flow andcause reduced operating voltages.

(3) Identification of possible jamming sources.Jamming stations use various types ofemission/modulation. When such interference isencountered, the possibility of jamming must beconsidered. Methods of jamming typically employed areas follows:

(a ) White noise. So-called "white" noisederives its name from its counterpart in the opticalspectrum, white light, which results from all frequenciesin the visible spectrum. Likewise, white noise consistsof many radio frequencies combined whose energycontent is relatively constant per unit bandwith acrossthe RF spectrum. Figure 4-2 contains oscilloscopetraces which show the effect of white noise interferenceon an analog signal for the conditions of the signal muchgreater than the interference, the signal slightly greaterthan the interference, and the interference much greaterthan the signal. Characteristically, white noiseinterference at levels equal to or greater than thedesired signal would completely block out any audiosignal with a continuous hissing sound. This type ofnoise is normally associated with jammers; however,when it occurs, the possibility of other problems, such aswhen semiconductors degenerate, should not be ruledout. At levels much less than the desired signal, whitenoise affects the signal as a background hiss. Whitenoise at considerably less amplitude than the signalcould point to circuit problems also and would notnormally be indicative of jamming.

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Figure 4-2. Examples of oscilloscope traces of I kHz test tone signal to white noise interference levels.

(b ) Random noise. An oscilloscope tracing of thistype of interference, which is similar to white noise, isshown in figure 4-3. At a jamming station, the noise isoriginated from the modulation of a jamming transmitterby a random noise generator. While not "white" in thesense that the energy is not

as evenly distributed throughout the electromagneticspectrum as white noise, the effect is much the same.The effect shown in figure 4-3 is that of random noise atapproximately the same power level as the analogsignal and would be heard as a hissing soundcompletely blocking out the signal.

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Figure 4-3. Examples of oscilloscope traces of an analog signal with various types of interference.

(c) Sawtooth. A sawtooth waveform isfrequently used to modulate a jammer. Figure 4--3depicts one example of interference causes by asawtooth waveform. It is unlikely that this type of

interference would be associated with any other causethan jamming. This interference would be heard as aperiodically occurring chirping sould which obliteratessignal intelligence.

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(d ) Voice. Voice interference can occur, ofcourse, as a result of crosstalk, or from othertransmissions; however, there is a possibility of theinterference being the result of a voice-modulatedjammer. Figure 4-3 shows the oscilloscope display as aresult of such a jammer signal interjected with an AMsignal. The sound of such a signal at the receiveroutput would be that of two signals being heard at thesame time. If the signal is strong and no known friendlystation is nearby, jamming should be suspected. If theinterfering signal is weak, fades irregularly, or occursonly at a certain time of day, it is more likely the resultof ionospheric refraction from a distant transmitter (HFand low end of VHF). Any interfering signal, digital orvoice modulated, should be compared with normaltraffic through the station. Friendly interferencecommonly encountered should also be compared.

(4) If jamming is suspected.(a ) Determine frequency of the interference.

Use a spare receiver connected to a separate antenna.or a battery powered multiband radio to determinewhether the interference is tunable; that is, whether theinterference appears on the desired signal RF channelonly, on adjacent RF channels, or at an imagefrequency. To determine whether transmissions at animage frequency are responsible, tune the radio aboveand below the desired signal tuned frequency by anamount equal to twice the IF of the station receiver.The characteristic sound of the interfering signal shouldincrease at one or the other of these settings if theimage frequency from another transmitter is involved. Ifthe interference is found to be cochannel with thedesired signal, a strong possibility exists that theinterference is intentional.

(b) Determine direction of interference. Use adirectional antenna in conjunction with the receiver orradio of step a above to identify the direction from whichinterference is coming. Based on the results of steps aand b, check with the appropriate authority to determinewhether a friendly station would normally be operatingfrom that direction and on the frequency encountered.

c. Isolation Procedures. The first step in theisolation of an interference problem is to examine thedegree to which the interference has affected thecommunications system; that is, how many subscribersare reporting problems. Next, the problem must beisolated to a major system, then further isolated to amajor component within the system. These proceduresare given in this order in the paragraphs that follow.

(1) Isolation based on extent of interference.The following procedures are based on whether one ormultiple subscribers being serviced by a particular

multiplex equipment are affected by the interferenceproblem.

(a ) Single subscriber affected. If theinterference affects a single subscriber, the troublesource is likely to be between the subscriber unit and thecommunications site or the appropriate patching facilityat the site. However, a single-channel fault could alsobe the source of interference. A typical isolationprocedure is as follows:

1. If the link to the subscriber is by cable, firstsubstitute spare subscriber equipment, including securitydevices. If interference is still present, check the cablein accordance with paragraph 4-3c (3) (b ).

2. If the subscriber is linked by radio, the outputof the subscriber's receiver must be checked todetermine whether the problem originates within orexternal to the subscriber station. If the receiver outputis normal (correct signal content and level) theninterconnecting cables and end-instruments may befaulty. Noise must be checked, substituting spareequipment, if available, to maintain service. If theinterference is introducted by RF means (para 43b (1))and no interfering station can be discerned, steps mustbe taken to determine whether a fault exists in thedistant transmission. Check in accordance with theprocedures described in paragraph 4-3c (2).

(b ) Multiple subscribers affected. If theinterference affects multiple subscribers, checks willdepend upon whether or not interference is present onthe incoming signal. Isolate as follows:

1. Incoming signal interference free. If aproblem affects various subscribers of the local station.and the distant station has checked and confirmed thattheir outgoing transmission is correct. and no otherexternal interference can be detected, the interferenceis local in origin, most likely occurring within the stationdemultiplexing equipment at the group level. Todetermine, check the input to the particular groupdemultiplexer in question in accordance with paragraph4-3c (2) (d 1). If the signal is clear, the problemoriginates in the demultiplexer equipment. If theinterference is present. check the preceding patchesand terminals in accordance with paragraph 4-3c (3) andalso check for the usual problems of loose connectionsand corrosion.

2. Incoming signal contains interference. If acheck of the incoming signal reveals that interference ispresent, and if the interference affects multiplesubscribers, the trouble most likely originates in the RFlink with the preceding station or in that station's RFequipment. Consider a typical example where a carriersystem employs voice frequency telegraph over a 4-kHzaudio channel and

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TM 11-490-5multiple subscribers are reporting problems. To affectrapid isolation of the source of the interference, proceedas follows:

(a) On the orderwire circuit, request thedistant-operator to check his transmission from thetransmit voice frequency telegraph system. This isaccomplished through appropriate patching andmonitoring and the results are reported. If the report isthat clear copy is leaving his station, then the troublelies either along the propagation path, in antennas andconnections, in any interconnecting cables, or within thelocal operator's own station.

(b) Monitor adjacent channels todetermine whether they are also affected. If thesechannels are affected, service may be restored byconnecting to available spare channels. Notify thedistant operator to do the same, after informing thesubscribers that a momentary interruption of servicemay be necessary while patching is accomplished.Equipment (possibly multiplex) malfunctions areindicated; or if only a few channels are affected,possible EMC problems are involved. If a singlechannel only was found to be affected, this indicatesthat other than EMC problems are involved andcorrective maintenance must be accomplished.

(c) A quick check can be made todetermine whether a carrier channel is at fault byreconnecting the channel at both stations to a sparechannel. Care must be taken that such reconnecting iscoordinated to avoid undue disruption of service.

(d) If connection to spare channelsdoes not restore service, the interference may becaused by a local transmitter or ancillary, or there maybe a problem arising from proximity of the transmissionline/antenna system to the circuit suffering theinterference. In this case, a detailed check of theantenna and transmission line would be required,including measuring the voltage standing wave ratio,unwanted ground connection, corrosion, insulationbreakdown, and the like.

(e) If problems are still apparent,checks should be made of the RF signal levelsthroughout the system, back to the originating station.Levels which are too high will usually cause overdriveproblems with accompanying nonlinear distortion.Those which are too low cause unnecessary degradationof the signal-to-noise ratio. Adjustments must be madeto ensure that levels are correct. Refer to paragraph 4-3d (8).

(f) It is possible that the problem couldstem from nonlinearities within the voice frequencytelegraph terminal, from the carrier system, or within theinterconnecting cables. A distortion analyzer may beused to detect nonlinearities in the system.

(g) If reconnection to a spare channel

fails to clear up the problem, equipment problems, suchas receiver alignment or receive module malfunctionsare possible. Isolate the trouble by substitution.

(c) If a station has been informed thattheir transmissions are interferring with the receipt ofsignals by another station, certain steps may be taken atthe suspected station to determine whether this isactually the cause of the problem:

1. Obtain approval to take the transmitter offthe circuit.

2. Terminate the transmitter into a dummyload, and proceed to check whether transmitter isoperating normally as indicated in paragraph 4-3c(2)(b).

(2) Major system isolation procedures. Asindicated previously, a major point where interferencecan enter a communication system is the radio link.This radio link normally consists of a transmitter withsome type of modulator, the receiver, the transmit andreceive antenna system, and the transfer mediabetween the two antennas.Although the exact methodology will vary dependingupon the equipment and type indicators or testinstrumentation available, the procedure is similar for allcomponents.

(a ) Receiver.1. With the test instrumentation available,

check the receiver output signal to determine if it isnormal for the particular system. If the system istransferring digital signals and the pulse recognition andpulse restoration circuits (that is, the circuits thatdetermine when a pulse is present and then generate atrue pulse at that time interval) are an integral part ofthe receiver, the signal should be monitored betweenthe detector and these pulse circuits. If the signal isnormal, the interference is not entering through the radiolink.

2. If the signal is not normal at the receiveroutput, the receiver input signal should be checkedusing available signal indicators, test instrumentation,and test points. If the RF signal cannot be checked, theIF signal can provide an indication of signal level anddeviation from normal. A spare receiver or broadbandportable radio may also be used to check the incomingRF signal. If the input signal is normal, as determinedby tests on the actual receiver, the spare receiver, or theportable radio, the distant transmitted signal isinterference free. If not, then the distant transmitsystem may be at fault. If there is no fault at the distantstation and there are no apparant propagation problems,local testing will be required to determine whether theproblem is associated with the actual receiver orreceiver antenna system. This can most readily beaccomplished by replacing the receiver. If the

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receiver output is now interference free, the receiverwas at fault. If the signal is still not normal the receiverantenna system should be checked. If available, useanother receive antenna system on circuit to minimizecircuit downtime during testing.

3. The above procedure is closely related tothat described in paragraph 4-3b (1) (a ), "Determineorigin of the interference." Both are examples of similarisolating techniques. There are other variants.

(b) Transmitter. Using the orderwire,operational personnel at the distant transmitter stationshould be requested to check whether transmitteroperation is normal. Generally, transmitter checks arelimited to assuring that modulation is normal and at theproper level and that all monitoring indicators andinstruments are reading normally (for example, the drivelevel is correct, the transmitter is on the correct antenna,and VSWR readings are normal). At some stations,spectrum analyzers are available, permitting visualobservation of the output signal. A spare receiver orbroadband portable radio with proper input attenuationto prevent receiver overload or damage can provide anindication of proper transmitter output, to include thetransmitting antenna system. If the transmitter output isnormal, the interference problem is associated with theantenna systems or the transfer media.

(c ) Antenna.1. Generally, transmitting antenna

problems will be indicated by changes in the readings ofthe output and loading at the transmitter. Interferenceproblems associated with receiver antennas generallywill cause fluctuations in the receiver signal level.However, interference in the incoming RF signal cancause similar indications. Interference problems couldarise from nonlinearities in multichannel HF antennaisolation or multicoupler devices.

2. If antenna problems are suspected,another antenna can be substituted, or repair personnelcan be requested to make a voltage standing wave ratio-(VSWR) or noise power ratio (NPR) test on the antennasystem.

3. If the distant transmitter output andthe antenna systems are normal, the interference isentering through the RF medium. A spare receiver orbattery operated broadband portable radio with sometype of directional antenna is probably the most rapidmeans of isolating the source, direction, and type ofinterference. The experience and capability of theoperating personnel are most important in therecognition of the type of interference and its probablesource. If appropriate monitoring equipment isavailable, the logical procedure when

interference is present in the receiver output would be tofirst check to determine whether the assigned RF isinterference free prior to conducting any further isolationtests.

(d ) Multiplex equipment. In simple terms,multiplex equipment combines multiple input signals(normally discrete audio channels) into a compositesignal that is used to modulate a transmitter without anyinterchannel interference and reverses the process atthe receiver by converting the composite received signalinto multiple channel output signals. To isolate aninterference problem to the multiplex equipment, theoutput and input signals are checked to determinewhether they are normal. If the input is normal and theoutput has interference, the problem is in the multiplexequipment. If the system uses double multiplexing(group and supergroup), composite input/output signalsof each of the multiplex elements should be checked ina similar fashion to that described above. Generally,grounding problems or direct external interferenceaffecting a multiplex equipment will interfere with alloutput channels of the specific multiplex element. Ifonly one output channel has interference, the mostprobable cause is a component malfunction within theequipment or a misadjusted drive level. Individualchannel drive levels are normally very critical on thetransmit multiplex. Check to be sure these drive levelsare adjusted to specified values. In mostcommunication systems some interference (noise andtines) is usually present. In a well-designed system, itwould be usual for interfering noise to be lower in levelthan the system's intrinsic noise and thus could beignored. However, if noise levels increase abovepredetermined levels, action is necessary to determinewhether a maintenance problem or an interferenceproblem exists. Up to a point, isolation procedures forboth types of problems are identical and along the linesdescribed above. Further information which will assistin isolating the problem is as follows:

1. Type of noise.(a) Whether the noise is wideband or

narrowband.(b) The frequency range of the

interference.(c) Whether a tone is involved and

whether the tone appears in more than one channel.Also, whether the tone is a modulated frequency of atone in the channel.

(d) Whether the interference is a signalinterference such as music, voice, on-off keying.

(e) Whether the noise seems to bepowerline related.

2. Levels.(a) Whether the receive signal power

levels are normal.

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TM 11-490-5

(b) Whether the transmit signal powerlevels are normal.

(c) Whether the correct loadingattentuation for the number of channels in operation hasbeen introduced.

3. Stations and systems affected.(a) Whether similar systems in the

station are affected.(b) Whether similar systems in other

stations are affected.(c) Whether a few stations or many

stations are affected.(d) In the case of dual-redundant

systems. the effect of turning "A" side and "B" side off.(e) Whether the noise is affected by

weather: for example, whether it is greater during dryweather.

(f) 'Whether any station reportedhigher than normal receive levels. In this case,waveducting (a propagation phenomenon) may beinvolved.

4. Special EMC considerations. Becausethe frequencies used by the internal processing portionof FDM multiplexing systems fall within the standard AMbroadcast band (535-1605 kHz), interference may resultwhen FDM systems are located near a broadcaststation. Typically, there are two different types ofinterference which may be experienced from.broadcast station sources. The first type results whenthe broadcast station assigned frequency falls in themiddle of an FDM channel. In this case, a serioussingle-tone interference caused by the mixing of thebroadcast station and FDM channel carriers isexperienced on one channel. However, the modulatingfrequencies of the broadcast signal can cause highnoise also in the two channels on either side of thechannel worst affected. As there is a frequency offsetbetween the two carriers, the noise will usually begarbled. Further, the noise on the channels will vary withthe broadcast station modulation. The second type ofinterference results from the broadcast station beingassigned a frequency that falls between two FDMchannels, resulting in a coherent signal being heard asnoise on one channel. A second channel will haveapproximately the same noise reading, but being on theother side of the broadcast frequency, will have invertedfrequencies. Usually, the higher modulating frequenciesat the broadcast station (above 4 kHz) will cause somenoise on one high and one lower channel, resulting inhigh noise on four channels. When interference from thebroadcast band is considered, it might be expected thata single broadcast station would affect four or fivechannels in a 600-channel FDM system. While this may

occur when the interference is introduced into thebaseband portion of the system, if introduced beforetranslation is completed, it is possible for interferencefrom one broadcast station to adversely affect four orfive channels in each basic supergroup in the system.For a 600-channel system, there are 10 basicsupergroups. Consequently, a single broadcast stationhas been known to adversely affect 40 to 50 FDMchannels. Introduction of broadcast band interferencecan result from many different factors. some of whichare-

(a) Equipment design deficiencies: poorgrounding inside the equipment, filters not effective atbroadcast band frequencies, and poor selection ofinterior cables.

(b) Equipment improperly grounded orgeneral grounding system not effective at higherfrequencies.

(c) Isolation transformers improperlyused.

(d) Design deficiencies in cableterminations.

(e) Cables improperly installed.(e) Security devices. Internal checks on

security devices are beyond the scope of this manual. Ifthe input of a security device appears normal and theoutput abnormal, unit substitution or checking groundwiring is the normal isolation procedure. If the deviceis multiple channel, interference associated with theunit would normally affect all channels. If a singlechannel is being interfered with, the problem is probablynot associated with the security device.

(3) Major component isolation procedures.(a) Switching or patching facilities. Since the

switching and patching facilities are normallynonautomatic, interference problems are generally dueto loose connections, worn cables, dirty contacts,improper grounding, incorrect patching or jumpering,and similar problems that can be isolated and resolvedby careful operation and proper maintenance.

(b) Interconnecting cables. The key toisolation of multipair cable problems is to check for anynew cables which may have been laid which could haveresulted in damage to existing cables, newly assignedcircuits in existing cables, and any vehicular traffic,usually in the vicinity of the station, which could havedamaged the cables. Such traffic can cause damage tothe cable insulation and -puncturing of the protectivesheath, resulting in moisture seepage into the cable.Eventually, this moisture can cause internal (interpairand interpair) corrosion which leads to partial shorts,leaks, or a path to ground which may result in groundloops, crosstalk, and hum. The test procedure for

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TM 11-490-5

identification of cable grounding problems consists ofdisconnecting the cable between the sites andmeasuring the resistance between pairs and theresistance to ground for each cable pair. A suitableohmmeter, impedance bridge, or megger can be usedfor this test. Interpair resistance indications of less than10,000 ohms per loop mile or resistance readings toground of less than 50,000 ohms is normally indicativeof a faulty cable or cable pair. Coaxial cables are verysusceptible to damage caused by heat, compression, orabrasion. When these occur, the results can bemismatches, loss of desired signal, or introduction ofinterfering signals. A floating shield caused by damageto the cable may result in unwanted signals beingintroduced into or radiated from the equipment or cable.

(c) Power line. If a battery operated,multiband radio is available, use it to trace the source ofpowerline problems, particularly if a defective insulatoror transformer is suspected as the source. Usually,such interference will increase in intensity as the sourceis approached with the radio. Poor ground connectionsat the power poles can result in a power frequency humin the communications equipment as a result of thecommon impedance in the ground loop. A rapid meansof isolation, if this is suspected. particularly in dry,sandy soil, is to thoroughly wet the ground around powerpoles close to the station with a saline solution. If theproblem .is powerline grounding, the interference will bereduced or eliminated. In the case of a broken insulator.hitting the suspected pole with a sledgehammer willcause a variation in the interference.

d. Remedial Procedures. The procedures whichoperating personnel can use in the correction ofinterference problems may be limited by available testequipment and maintenance level authorization. Thefollowing procedures, where authorized. may be usedfor the correction of the corresponding interferencesources given in paragraph 4-3b (2) (b ):

(1) Ignition noise.(a) Identify the source of the ignition

noise to the specific source such as privately-ownedvehicles, lawn mowers, or similar gasoline-poweredengines.

(b) Report this information to the levelwhere remedial action can be taken. Such action mayinclude restriction of the ignition noise sources from thecommunication site, installation of noise suppresserdevices, or restricting their use to periods of lowcommunications usage.

(2) Radar. Radars are sometimes locatedwhere they will sweep the area of a communicationssite. In the event that radar interference is encountered,the following procedures should be followed.

(a) Report interference to properauthorities.

(b) Initiate action for possible sectorblanking of the radar signal or other reduction in theradiation of this signal.

(c) Relocate either the communicationssite or the radar, if practicable. In some cases, thesolution could be as simple as a relocation of receiveantennas.

(3) Office equipment. Qualified repairmenmust be consulted to eliminate problems from thesemachines, including the assurance that the internalground connections are properly maintained.Procedures follow:

(a) Identify specific equipment causingproblems. A 3-wire power cord properly grounding theequipment may solve the problem. If it does not,

(b) Report interference.(c) Initiate request for shielding or

relocation.(4) Power lines.

(a) Isolate the cause of interference tothe most likely source, such as a power transformer orinsulator, with procedures given in paragraph 43c(3)(c).

(b ) Report this information to theresponsible authority, such as the local facility engineer.

(5) Cables. Damaged cables must bereplaced, or spare cables used. When the cause of thecable damage has been determined, appropriate stepscan be taken to ensure that it does not happen again,including the erection of signs and barriers to preventvehicular traffic over the cables. Corroded and looseconnections can be corrected by cleaning andtightening. If this is not possible, reroute to anothercable pair, if available.

(6) Circuit components. Problems whichhave been traced to faulty circuit components, such asline amplifier modules. will generally require theservices of a repairman to test at the appropriatemonitoring jack and replace or adjust as necessary.Patching may be required to bypass the particularequipment involved while this maintenance is beingperformed.

(7) Equipment grounding. All electricallyoperated equipment within the site must be maintainedat ground potential by interconnection with the commonstation ground. At DCA facilities, a station ground mustbe maintained at the DCS standard and periodicallymeasured by the supporting maintenance or engineeringfacility. Non-DCS transportable or mobile installationsalso require good grounds. but have special groundingproblems associated with their temporary deployment.Grounding rods furnished with the equipment may notsuffice in some areas. In sandy soils. use of buriedcopper plates to increase the contact area with the soilmay be necessary. Also, all grounds may requireregular attention in accordance

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with local instructions, such as soil treatment with achemical solution, if necessary. Special emphasis mustbe directed to the use of 3-wire grounding power cordsfor ancillary equipment and hand tools with the groundwire connected to the station ground.

(8) RF signal level. Intermodulation andcrosstalk interference problems attributable to highsignal level will require adjustment to eliminate theproblems. Any attempt to adjust a signal level at thereceiving station without a check of the levelsthroughout the system, however, may not solve theproblem if it is caused by high signal level at channel,group, or supergroup levels of a previous station whichhas resulted in baseband overloading. Signal leveladjustment can be undertaken while the channels are inservice, depending on the severity of the interferenceproblem, but usually this adjustment must be done outof service with test tones applied.

(9) Interfering transmitters. If it is determinedthat a station's own transmission is causing excessiveinterference to others and circumstances preclude afrequency change, a sideband change, or other similarsolution, the interference problems could be reduced bya slight reorientation of the transmit antenna (if this ispracticable) to move the interfering lobe away from thedistant station suffering the interference. Anotherpossible solution could be to adjust the beam shape.For example, if a "sloping V" antenna configuration isbeing employed as an HF transmitting antenna, it maybe possible to decrease the apex angle of the antennato an optimum value of 30 degrees. The effectivechange in beam shape may avoid transmission into anearby receiver. Suggestions for such a change shouldbe referred to senior station engineering staff.

(10) Antennas and transmission lines. Loosebroken and corroded antenna and waveguideconnections and tower guywires are a source of noisewhich can be reduced through regular periodicinspection, cleaning, tightening, and other maintenancemeasures. This is an important facet of installation andmaintenance. For example, copper wire strung duringhot weather conditions can become taut and snap duringcold conditions, if it was too tightly strung. Antennaorientation can sometimes be varied slightly to avoidinterference pickup. Waveguide pressurization shouldbe periodically checked in accordance with theapplicable specifications. In the case of opentransmission lines, proper connection at installation,orientation in relation to possible noise sources, andconstant line spacing are important to the reduction ofinterference problems.

e. Follow-On Procedures.(1) The above procedures for the recognition,

isolation, and remedy of EMC problems should besufficient to detect and correct most of the less complexinterference problems that normally occur to fixed andtransportable facilities. Persistent, chronic occurrencesof EMC problems, particularly those which may occurupon activation of new systems within a site, will requireresolution by the supporting maintenance or engineeringassistance personnel. However, it is essential that siteoperating personnel learn to recognize both EMC andECM problems and to distinguish between them.

(2) Reporting procedures are contained in thefollowing chapter.

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CHAPTER 5

PROCEDURES FOR REPORTING OPERATIONAL EMC PROBLEMS

5-1. GeneralIn the event that interference is encountered, it must bereported to proper authorities in accordance withinstructions provided herein. Prior to submitting areport, the unit or facility concerned will:

a. Conduct an initial evaluation to explore thepossibility of locally generated spurious signals or othertechnical difficulties which might have caused theinterference.

b. Without delay, coordinate with colocated unitsor those in nearby areas to isolate possible sources. Anycoordination undertaken should not delay submission ofthe initial incident report, which is required within 24hours. The reporting unit will continue to investigate theincident after submission of the initial report. Anyadditional pertinent information discovered during thiscontinuing investigation will be reported promptly to allrecipients of the initial report.5-2. ProceduresThe following procedures will be used to insure Armycompliance with (C) AR 105-3, "ReportingMeaconing, Intrusion, Jamming, and Interference (MIJI)of Electromagnetic Systems" (U).

a. Commands may issue supplemental instructionsand procedures for the reporting, analyzing, evaluating,and disseminating of information concerning MIJIincidents.

b. Commands may abbreviate local formats toinclude only appropriate items in accordance withattachment to AR 105-3; however, the samenumber/letter designation as shown must be retained.

c. The affected unit or facility will provide the initialincident information by a dual-addressed message, viasecure communications means through C-E channels,for action to the local USASA support unit and the localUSACC/USACEEIA activity. Additional addressees maybe included as considered necessary by the command.The purpose of this procedure is to alert the appropriatecommand echelon which has the necessary resources toassist in further identifying and/or resolving the problemlocally. d. Upon determination by the command that theincident was probably hostile or, in the case ofinterference, was not readily identifiable, a formal MIJIreport will be forwarded within 24 hours per instructionscontained in AR 105-3. To reduce the reporting

requirements of the affected operational unit or facility,it is recommended that the action addressees of theinitial incident information message submit the formalMIJI report as a coordinated action and provide anyadditional pertinent information in the remarks section tomore fully document details surrounding the incident.

e. Upon determination by the command that theinterference was clearly unintentional and beyondresolution within the command, the frequency managerwill initiate an interference report, in the format of theattachment to AR 105-3, to higher frequencyassignment authorities in accordance with policies andprocedures established by ACP and US SUPP-1 ( ),"Frequency Management;" and ACP 121 and SUPP-1(), "Communications Instructions, General." Informationaddressees will include AFSPECOMMCEN, Kelly AFB,Texas; Commander, US Army CommunicationsCommand, ATTN: CC-OPS-OS; and Commander, USArmy Communications-Electronics EngineeringInstallation Agency, ATTN: CCC-CED-EMEA, FortHuachuca, Arizona 85613.

f. Upon determination by the command that theinterference was a technical or operationalelectromagnetic incompatibility problem, pertinentinformation will be forwarded to US ArmyCommunications-Electronics Engineering InstallationAgency (USACEEIA), ATTN: CCC-CED-EMEA, withinformation copy to Commander, US ArmyCommunications Command (USACC), ATTN: CCOPS-OS, Fort Huachuca, Arizona 85613:

(1) For information-when the problem isresolved within the local command.

(2) For action-when the problem is beyondresolution within the local command. USACEEIA willcoordinate the assistance required to solve operationalEMC problems in accordance with assignedresponsibilities under the operational electromagneticcompatibility area of the Army EMCP (AR 11-13).Information copies will be forwarded to all concernedand include AFSPECOMMCEN, Kelly AFB, Texas.

g. Commands will forward a copy implementinginstructions to: Commander, US Army Communications-Electronics Engineering Installation Agency, ATTN:CCC-CED-EMEA, Fort Huachuca,

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Arizona 85613; Commander, US Army Security Agency,ATTN: IAFOR-E, Arlington Hall Station, Arlington,Virginia 22212; and the Air Force SpecialCommunications Center, Kelly AFB, Texas 78241.

h. Requests for other assistance involvingoperational EMC problems, electromagneticenvironmental surveys, and potential hazards toammunition and fuel handling storage sites should be

addressed to Commander, USACEEIA, ATTN: CCC-CED-EMEA. Requests for assistance involvingsuspected or potential electromagnetic radiation hazardsto personnel should be addressed to the US ArmyEnvironmental Health Agency, ATTN: HSE-RL,Aberdeen Proving Ground, Maryland 21005, with aninformation copy to Commander, USACEEIA, ATTN:CCC-CEDEMEA, Fort Huachuca, Arizona 85613.

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TM 11-490-5

APPENDIX

DEFINITIONS

Absorption--Wave propagation energy losses due toconversion of wave energy to heat.

Adjacent channel, audio--An audio channel immediatelyabove or below a specified audio channel.

Adjacent channel, RF-An RF channel immediatelyabove or below a specified RF channel.

Analog system--A system of communications in whichinformation is transferred in the form of acontinuously variable waveform.

Arcing--A low-voltage, high current electrical discharge,as contrasted with sparking.

Amplitude modulation (AM)--A method of modulating acarrier wave to cause it vary in amplitude accordingto the modulating intelligence.

Bandwidth--The difference, expressed in Hz, betweenspecific upper and lower limiting frequencies whichare usually specified as the upper and lower half-power (3 dB) points of a system or network.

Binary--A system of numbers having two as its base.Used in digital data transmission systems andcomputers.

Broadband emission--An emission which has a spectralenergy distribution sufficiently broad and continuousso that the response of the measuring receiver in usedoes not vary measurably when tuned over aspecified number of receiver bandwidths.

Capacitive coupling--The means by which signals aretransferred from one conductor to another throughthe capacitance existing between the conductors.

Characteristic curve--As applied to transistors andelectron tubes, the relationships between the variousvoltages and currents as evidenced at the externalconnections; for example, the curves of base currentversus collector current for a transistor.

Cochannel interference--Electromagnetic interferencecaused in one communication channel by atransmitter operating in the same channel. Corona-The discharge brought a out as a result of theionization of air (or gas) surrounding a conductor.

Cross modulation--A type of Intermodulation due to themodulation of the carrier of the desired signal by anundesired signal wave. Occurs primarily as a resultof nonlinearities in the RF circuitry of a receiver.

Desensitization--The effect of an undesired signal in thereceiver band pass which causes reduction of the

desired signal level. The gain reduction is generallydue to overload or saturation of some portion of thereceiver. Can also occur when an interfering signalcauses undesired operation of the AGC system in thereceiver.

Dielectric--An insulating material which essentiallyprohibits the flow of electrical current betweenconducting components of a network.

Dual-conversion receiver-A receiver in which RF to IFconversion is accomplished in two steps, employingtwo local oscillator/mixer systems and two IFamplifiers.

Decibel (dB)--A logarithmic unit used incommunications work to express power ratios.

Diffractions--The process by which electromagneticwaves curve around edges and penetrate into theshadow region behind an opaque obstacle.

Digital system--A system of communications in whichinformation is represented by a series of discretesignal elements or digits (binary or other).

Electromagnetic compatibility (EMC)--The ability of C-Edevices, together with electromechanical devices, tooperate in their intended operational environmentwithout suffering or causing unacceptabledegradation because of unwanted electromagneticenergy. EMC includes vulnerability to ECM.

Electromagnetic interference (EMI)--Interference is anyelectromagnetic emission causing undesirableresponses which degrade, disturb, or disrupt thedesign function of devices or systems which employelectromagnetic energy. For frequency managementpurposes, the term "harmful interference" is used todenote any emission, radiation, or induction whichendangers the functioning of a radionavigationservice or of other safety services, or seriouslydegrades, obstructs, or repeatedly interrupts a radiocommunications service operating in accordancewith international regulations. Additionally, the term"harmful interference" is used to denote that type ofinterference which actually causes circuit outage asopposed to interference which is purely a source ofannoyance.

Electronic Warfare (EW)--Military action involving

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the use of electromagnetic energy to determine, exploit,reduce, or prevent hostile use of the electromagneticspectrum, and action which retains friendly use of theelectromagnetic spectrum. There are three divisionswithin EW:

Electronic Warfare Support Measures (ESM): Thatdivision of EW involving actions taken to search for,intercept, locate, and immediately identify radiatedelectromagnetic energy for the purpose of immediatethreat recognition.

Electronic Countermeasures (ECM): That division ofEW involving actions taken to prevent or reduce anenemy's effective use of the electromagnetic spectrum.ECM includes:

1. Jamming. The deliberate radiation, radiation, orreflection of electromagnetic energy with one object ofimpairing the use of electronic devices, equipment, orsystems being used by an enemy.

2. Deception. The deliberate radiation, radiation,alteration, absorption, or reflection of electromagneticenergy in a manner intended to mislead an enemy in theinterpretation or use of information received by hiselectronic systems. There are two categories ofdeception:

a. Manipulative. The alteration or simulationof friendly electromagnetic radiations to accomplishdeception.

b. Imitative. Introducing radiations intoenemy channels which imitate his own emissions.

Electronic Counter-Countermeasures (ECCM): Thatdivision of electronic warfare (EW) involving actionstaken to insure friendly effective use of theelectromagnetic spectrum despite the enemy's use ofEW.Emission spectrum--The power versus frequency

distribution of an emitted signal, including thefundamental frequency, associated sidebands, andall spurious emissions.

Frequency division multiplex (FDM)--A means by whichtwo or more channels of information are transmittedand received simultaneously without mutualinterference by virtue of assigning each channel aseparate frequency within the total availablefrequency band.

Group loop--In an electrical system, a potentiallydetrimental circuit path formed by theinterconnection. of two or more points that arenonimally at ground potential.

Harmonic frequency--An integral multiple of thefundamental frequency; that is, the frequencies of 2,3, 4, 5, ... times the fundamental frequency.

Heterodyne-The combining (beating) together in anelectrical circuit of two frequencies to produce new

frequencies which are the sum and difference of thetwo original frequencies.

Image frequency--The frequency which differs from thetuned frequency of a receiver by an amount equal totwice the inermediate frequency (the firstintermediate frequency of a dual conversionreceiver) of the receiver. The image frequency canbe above or below the receive tuned frequencydepending on whether the local oscillator is setabove or below the receiver tuned frequency. Anunwanted signal at the image frequency maybecome an interfering signal.

Inductive coupling--The type of coupling in which themechanism is mutual inductance between theinterference source and the signal system; that is,the interference is induced in the signal system by amagnetic field produced by the interference source.

Intermodulation--The production of frequencies in theoutput of a nonlinear device equal to the sums anddifferences of the frequencies present at the input ofthe device, plus the sumsand differences ofharmonics of the input frequencies. For example, if fa and f b are the frequencies of the two input signals,the output signal could contain signals at frequenciesof fa , fb , 2fa , 3fa , 4fa. . . . , 2fb, 3fb, 4fb. . . . , fa ± fb, fa

± 2fb , fa ± 3fb. . . . , fb ± 2fa , fb ± 3fa , fb ± 4fb , and soforth; in general; nfa ± mfb , where either n or m couldbe 0, 1, 2, 3 and so forth. The signal componentsthus generated are sometimes called"Intermodulation products" The "order" of the productis n + m; for example, fa + 2fb is a third orderIntermodulation product.

Ionization--The process which results in the formation ofan atom, or molecularly bound group of atoms, inwhich one or more electrons have been gained orlost, resulting in an ion or ions, each having a netnegative or positive charge.

Jamming--See Electronic Warfare Local oscillator-Anoscillator, incorporated within the equipment, whoseoutput is mixed with another frequency waveform forfrequency conversion.

Magnetic field coupling--Identical with InductiveCoupling.

Nonlinear Element--An element in which therelationships between the output and input are notlinear; that is, the element alters the input (voltageand/or current) waveform and produces an altered ordistorted version of the input waveform. Someelements or devices are designed to be non-linear(such as mixers, detectors, and clippers) while someare linear under some conditions and nonlinear underother conditions (for example, a linear amplifieroperating under designed conditions versusoverdriven conditions resulting in waveformdistortion). Distortion causes harmonics

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TM 11-490-5

of the input frequencies to appear at the output. Forexample, if fa is the frequency of the input signal, theoutput signal could contain signals at fa , 2fa , 3fa , 4fa ,and so forth at various amplitudes. When two or moresignals are simultanesouly inputted to a nonlinearelement, the output will contain signals at frequenciesthat are the sums and differences of the fundamentaland harmonics of the frequencies of the input signals.(See Intermodulation.)Oscillator--A circuit that generates a periodic signal.Parasitic oscillation--An undesirable oscillation occurring

in RF amplifiers or oscillators, usually caused by thechange in impedance of circuit elements, atfrequencies remote from the normal operatingfrequencies.

Pulse repetition frequency (PRF)--In pulsed radar or anyother system employing periodic pulses, the numberof pulses occurring each second, expressed in Hz.

Receiver response--The manner in which a receiverreacts to wanted and unwanted signals.

Rectification--Essentially, the process of converting acto dc.

Refraction--The change of direction, or apparentbending experienced by an electromagnetic wave asit passes obliquely from one medium to another orthrough a medium having nonuniform density. Anexample is the wave bending effect which occurs asan electromagnetic wave passes through theionosphere. Saturation--The condition in any circuitthat exists when an increase in the input produces nofurther change in the output; for example, an electrontube driven into th saturation region by too large aninput signal.

Shielding--The surrounding of electrical componentswith a conducting medium, such as aluminum, toreduce the capacitive and inductive coupling ofundesired signals from one circuit to another.

Shot effect--A troublesome phenomenon which causessputtering or popping noises in electronic equipmentdue to variations in the number of electrons emittedper second from the cathode of an electron tube andinstantaneous variation in the distribution of theelectrons among the tube electrodes.

Single-conversion receiver--A receiver employing onelocal oscillator and one mixer stage for conversion toa single intermediate frequency (IF) as opposed to adouble conversion receiver which uses twointermediate frequencies.

Sparking--The sudden breakdown of the insulatingstrength of the dielectric separating two electrodes,accompanied by a rush of electrical current acrossthe "spark gap," and a flash of light indicating veryhigh temperature. Unlike the arc, the spark is of veryshort duration. Spurious emission-Any unwantedemission from a transmitter.

Spurious response--Any response of a receiver tofrequencies outside the designated receptionbandwidth.

Squelch--To automatically disable the audio output of areceiver when the incoming RF signal falls below apreset level, or is absent.

Susceptibility--The degree to which a device,equipment, or weapon system is open to effectiveattack due to one or more inherent weaknesses.Thermal agitation-Random movement of electronswithin a conductor as a result of heat which maycause unacceptable noise within the circuit.

Thermostatic device--A device which operates inresponse to temperature variations. An example is athermostatically controlled switch.

Time division multiplex (TDM)--A means by which two ormore channels of information are transmitted andreceived simultaneously without mutual interferenceby virtue of assigning each channel specific timeintervals within the total unit time available.

Transients--Momentary fluctuations of voltage andcurrent in a circuit. They may be the result of adisturbance such as opening or closing a switch, akeying apparatus, or noise sources.

Troposphere--A thermal atmospheric region, extendingfrom the earth's surface to the ionosphere,characterized by decreasing temperature with height,appreciable vertical wind motion, vapor content, andweather phenomena.

Vulnerability--The characteristics of C-E devices,together with electromechanical devices, whichcause them to suffer degradation of performance asa result of having been subjected to intentionalelectromagnetic interference in a manmade, hostileenvironment.

White noise--A complex wave consisting of many radiofrequencies. Its energy content is relatively constantper unit bandwidth across the RF spectrum.

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TM 11-490-5INDEX

Paragraph Page Paragraph PageAbsorption, effect on interference..................................... 2-3c(1) 2-10 Jamming....................................................................... 4-3b(1)(e) 4-4Adjacent channel operation: ......................................................... 4-3b(1)(f) 4-4

Determination...........................................................4-3b(1)(d) 4-4 ................................................................................... 4-3b(3) 4-7Interference effect ................................................... 2-2c(1}(a) 2-3 Common impedance coupling ......................................... 2-3b(1) 2-10Interference resulting ............................................... 2-4c(1)(a) 2-12 Conduction:Multiplexing techniques ....................................................3-2d 3-4 Transmitter signals ............................................. 2-2c(1)(c)4 2-8Result of intermodulation........................................ 2-2c(1)(c)4 2-8 Interference................................................................... 2-3b 2-10

Analog signals: ............................................................................. Control devices, interference ....................................... 2-4c(2)(b) 2-15Conversion to digital ..................................................... 3-2c(1) 3-2 Corona. on power lines ............................................... 2-2c(2)(a) 2-8Effect of jamming ........................................................ 4-3b(3) 4-4 Coupling:Interference effect .............................................................3-2b 3-2 Capacitive ................................................................. 2-3b(2) 2-10

Antenna system:........................................................................... Induction ................................................................... 2-3b(3) 2-10Isolation of problems ............................................... 4-3c(2)(c) 4-12 Interference entry mechanism.........................................2-4b 2-12Remedial procedures for problems ............................. 4-3d(9), 4-15, Mutual ....................................................................... 2-3b(1) 2-10

4-3d(10) 4-15 Transmitter signals ............................................. 2-2c(1}(c}4 2.8Amplitude modulation; effect Cross modulation:

on multiplex ...........................................................4-3c(2)(d)4 4-13 Contrasted with intermodulation ................................. 2-2c(1)(c)1 2-6Arcing:.......................................................................................... Equipment susceptibility............................................... 2-4c(1)(d) 2-14

In ignition systems ...................................................2-2c(2)(d) 2-9 Source of................................................................ 2-2c(1)(c) 2-4In rotating machinery.................................................2-2c(2)(f) 2-9 Crosstalk:In welding equipment .............................................. 2-2c(2)(c) 2-9 High signal levels .......................................................4-3d18) 4-15Arc welding equipment, In cables ................................................................... 4.3b(2)(b)5, 4-7,interference ............................................................. 2-2c(2)(c) 2-9 4-3c(3)(b) 4-13

Atmospheric noise, sources ............................................. 2-26(2) 2-2 Voice interference ..................................................4-3b(3)(d) 4-10Audio responses: ..................................................................... Decibel (dB), solar noiseAural indications .......................................................... 4-3a(2) 4-3 expressed .......................................................................2-23 2-2Cable interference ................................................ 4-3b(2)(b)5 4-7 Desensitization, result ofEffect of jamming ....................................................... 4-3b(3) 4-7 nonlinear operation ................................................... 2-4c(2() 2-15Hum due to poor grounds.......................................4-3b(2)(b)6 4-7 Dielectric, interference effectIgnition noise ..........................................................4-3b(2)(b)1 4-6 of film ......................................................................2-2c(2)(f) 2-9Nature of interference................................................. 4-3b(2) 4-4 Diffraction:Noise .......................................................................2-4c(1)(d) 2-14 Effect on ground waves ..............................................2-3c(1) 2-10Office equipment interference .............................. 4-3b(2)(b)3 4-6 Effect on skywaves .....................................................2-3c(1) 2-10Power line interference ......................................... 4-3b(2)(b}4 4-7 Effect on troposphericRadar interference .................................................4-3b(2)(b)2 4-6 scatter waves ............................................................ 2-3c(1) 2-10

Back-to-back" test, use ............................................ 4-3b(1)(b)7 4-4 Digital computers, interference..................................... 2-4c(2)(a) 2-15Baseband, interference isolation ..................................4-3b(1)(g) 4-4 Digital system:Broadband emission:.................................................................... Analog-to-digital conversion ............................................. 3-2b(1) 3-2

Interference sources .................................................... 2-2c(2) 2-8 Detecting interference.....................................................3-2c 3-2Random noise .........................................................4-3b(3)(b) 4-8 Error effect ........................................................................3-2c(2) 3-3White noise ..............................................................4-3b(3)(a) 4-7 Interference isolation at receiver........................ 4-3c12)(a)1 4-11

Broadcast band interference, Nature of interference ........................................... 4-3b(2)(a) 4-4effect on FDM ...................................................... 4-3c(2)(d)4 4-13 Susceptibility to interference .......................................3-2c(3) 3-3

Cabling: Displays, effect of interference..................................... 2-4c(2)(c) 2-15EMC problems caused .............................................. 4-2b(2), 4-2 Distortion:

4-3b(2)(b)5 4-7 In overdriven amplifier .......................................... 2-2c(1)(c) 2-4Isolation of problems ...............................................4-3c(3)(b) 4-13 In transistor circuit ................................................. 2-2c(1)(c) 2-4Remedial procedures ................................................... 4-3d(5) 4-14 Dual-conversion receiver,

Capacitor coupling: spurious response................................................ 2-4c(1)(c) 2-14Equipment susceptibility ...................................................2-4b 2-12 Ducting:Interference transfer through........................................ 2-36(2) 2-10 Effect ...............................................................4-3c(2)(d)31f) 4-13

Characteristic curve, nonlinear Propagation result ...................................................... 4-2b3) 4-2portion ...................................................................... 2-2c(1)(c) 2-4 Electromagnetic compatibility (EMC):

Circuit components, remedial Effect of multiplexingprocedures .................................................................. 4-3d(6) 4-14 on problems ...................................................................3-2d 3-4

Cochannel interference: Effect of signal typeDiscussion .............................................................. 2-2c(1)(a) 2-3 on problems .................................................................... 3-2 3-1Equipment susceptibility .......................................... 2-4c(1)(a) 2-12 Follow on of problems ....................................................4-3e 4-15AM broadcast stations........................................ 4-3c(2)1d)4 4-13 Isolation based on extent ........................................... 4-3c(1) 4-10Intermodulation..................................................... 2-2c(1)(c)4 2-8 Isolation of problems ........................................................... 4-3c 4-10

Index 1

TM 11-490-5

Paragraph Page Paragraph PageIsolation in major components...................................... 4-3c(3) 4-13 Incoming signal not free....................................... 4-3c(1)(b)2 4-10Isolation in major system............................................ 4-3c(2) 4-11 Intermediate frequency........................................... 2-4c(1)(b) 2-13Nature of problems..................................................... 4-3b(2) 4-4 Intermodulation and cross modulation.................... 2-4c(1)(d) 2-14Of DCS stations................................................................4-2b 4-2 Intersite, three causes................................................ 4-2b(3) 4-2Of non-DCS stations.........................................................4-2c 4-2 Isolation...........................................................................4-3c 4-10Origin of problems........................................................ 4-3b(1) 4-3 Multiple subscribers affected ................................ 4-3c(1)(b) 4-10Recognition of problems...................................................4-3a, 4-3, Remedial procedures ..................................................... 43d 4-14..........................................................................................4-3b 4-3 Single subscriber affected ................................... 4-3c(1)(a) 4-10Relation to equipment susceptibility ...................................2-4 2-12 Sources, manmade, broadband ..................................2-2c(2) 2-8Relation to interference ................. .....................................2-2 2-1 Sources, manmade, narrowband ................................2-2c(1) 2-3Relation to transfer media ...................................................2-3 2-10 Sources, natural..............................................................2-2b 2-1Relationship of problems.......................................................... Spurious response................................................. 2-4c(1)(c) 2-14to personnel......................................................................4-1b, 4-1, Unintentional versus intentional.......................................4-2d 4-2..........................................................................................4-1c 4-1 Intermodulation:Remedies for problems ................................................... 4-3d 4-14 Production............................................................ 2-2c(1)(c)2 2-6Reporting of problems..................................................5-1, 5-2 5-1, 5-1 Products.............................................................. 2-2c(1)(c)3, 2-7,Three factors.....................................................................2-1b 2-1 ............................................................................... 2-4c(1)(d) 2-14Two categories of problems ..............................................4-1a 4-1 Result of high RF levels ............................................. 4-3d(8) 4-15Unintentional versus intentional................................................ Result of nonlinearities........................................... 2-2c(1)(b) 2-3problems ...........................................................................4-2d 4-2 Ionosphere, propagation....................................................2-3c(2) 2-11

Electromagnetic interference (EMI) .............................................. Ionization:(See Interference) .................................................................... In ionosphere .............................................................2-3c(2) 2-11

Electronic warfare (EW)............................................................... Transients caused ................................................ 2-2c(2)(b) 2-9(See Jamming)......................................................................... Isolation procedures:

Emission spectrum:...................................................................... Antenna.................................................................. 4-3c(2)(c) 4-12Effect of intermodulation............................................2-2c(1)(c)(2 2-6 Major component.........................................................4-3c(3) 4-13Spurious emissions ........................................................ 2-2c(1)b 2-3 Major system...............................................................4-3c(2) 4-11Transmitter, check...................................................... 4-3c(2)(b) 4-12 Multiplex................................................................. 4-3c(2)(d) 4-12Equipment noise, sources.............................................. 2-26(1) 2-1 Receiver................................................................. 4-3c(2)(a) 4-11Fluorescent lights, interference .......... .........................2-2c(2)(b) 2-9 Transmitter............................................................. 4-3c(2)(b) 4-12Frequency allocation ....................................................2-2c(1)(b) 2-3 Jamming:Frequency division multiplex (FDM): ............................................ Caution concerning .................................................4-3b(1)(f) 4-4

Discussion ................................................................... 3-2d(1) 3-4 Identification..................................................... ..........4-3b(3) 4-7Effect of ignition noise........................................... 4-3b(21(b)1 4-6 If suspected ............................................................... 4-3b(4) 4-10Isolation of problems ................................................4-3c(2)(d) 4-12 Possibility ........................................................................4-2d 4-2Multiple multiplex.......................................................... 3-2d(3) 3-5 Reporting incidents ..........................................................5-2 5-1Galactic noise...................................................................2-24) 2-2 Sawtooth ...................................................................4-3b(3c) 4-9

Grounding: ................................................................................... Voice......................................................................4-3b(3)(d) 4-10Effect of defective ................................................ 4-3b(2)(b)6 4-7Isolation of trouble .................................................. 4-3c(3)(c) 4-14 Local oscillator:Remedy for defective.................................................. 4-3d(7) 4-14 Harmonics.............................................................. 2-2c(1)(b) 2-3

Harmonics, of transmitted frequency- ..........................2-2c()6(b) 2-3 Spurious response related ................................... 2-4c(1)(c) 2-14IF interference .............................................................. 2-4c(1)b) 2-13 Magnetic field couplingIgnition noise: ............................................................................... (See Inductive coupling)

Causes........................................................................ 2c(2)(d) 2-9 Maintenance, importance .................................................4-2a(1), 4-1,Effect........................................................................... 4-2b(2), 4-2,Remedy........................................................................ 4-3d(1) 4-14 ....................................................................................4-3a(1) 4-3

Image frequency, interference...................................................... Narrowband.......................................................................2-2c(1) 2-3effect ........................................................................4-3b(4)(a) 4-10 Sources.................................................................................2-2c 2-2

Industrial apparatus, interference .................................2-2c(2)(h) 2-10Inductive coupling: ....................................................................... Multiplex:Equipment susceptibility..........................................................2-4 2-12 FDM........................................................................... 3-2d(1) 3-4Interference transfer through............................................ 2-3b(3) 2-10 Isolation of interference ........... .............................4-3b(1)(g). 4-4,Interference: ................................................................................. ............................................................................... 4-3c(2)(d) 4-12

By conduction......................................................................2-3 2 -10 Multiple....................................................................... 3-2d(3) 3-5By radiation .......................................................................2-3c 2-10 Special considerations ....................................... 4-3c(2)(d)4 4-13Cochannel and adjacent channel ............................ 2-4c(1)(a) 2-12 TDM........................................................................... 3-2d(2) 3-5Determination of direction.........................................4-3b(4)(b) 4-10 Techniques ....................................................................3-2d 3-4Determination of frequency ......................................4-3b(4)(a) 4-10 Noise:Determination of nature................................................ 4-3b(2) 4-4 Atmospheric............................................................. 2-2b6(2) 2-2Determination of origin ................................................. 4-3b(1) 4-3 Equipment.................................................................. 2-2b(1) 2-1Effect on analog signals ....................................................3-2b 3-2 Galactic...................................................................... 2-2b(4) 2-2Entry mechanisms ............................................................2-4b 2-12 Ignition ................................................................... 2-2c(2)(d) 2-9,Equipment susceptibility......................................................2-4 2-12 .............................................................................4-3b(2)(b)1 4-6Incoming signal free...............................................4-3c(1)(b)1 4-10 Intermodulation....................................................... 2-4c(1)(d) 2-14

Index 2

TM 11-490-5

Paragraph Page Paragraph PageIn test equipment......................................................2-4c(2)(d) 2-15 Ignition noise.............................................................. 4-3d(1) 4-14Random.................................................................. 4-3b(3)(b) 4-8 Office equipment interference .................................. 4-3d(3) 4-14Solar ............................................................................ 2-2b(3) 2-2 Powerline interference ............................................... 4-3d(4) 4-14White...................................................................... 4-3b(3)(a) 4-7 Radar interference ................................................... 4-3d(2) 4-14

Nonlinear element, contribution to ................................................ RF signal level adjustment ...................................... 4-3d(8) 4-15interference .............................................................. 2-2c(1)(c) 2-4 Transmitter, interfering............................................. 4-3d(9) 4-15

Normal indications, personnel ...................................................... RF signal level:familiarity with...................................................................4-1c, 4-1, Adjustment ................................................................ 4-3d(8) 4-15..................................................................................... 4-3a(1) 4-3 Result of incorrect............................................4-3c(1)(b)2(e) 4-11

Office equipment interference: ..................................................... Rotating machinery, interference ..................................2-2c(2)(f) 2-9Effect........................................................................ 4-3b(2)(b)3 4-6 Saturation, result ofRemedy ............................................................................ 4-3d(3) 4-14 nonlinear operation................................................. 2-4c(2)(b) 2-15Operating personnel: .................................................................... Security devices ........................................................... 4-3c(2)(e) 4-13Importance of experience ......................................... 4-3c(2)(c)3 4-12 Shot effect, in equipment..................................................... 2-2b 2-1Knowledge of equipment performance ............................ 4-2b(1), 4-2, Signal tracing, to isolate

.................................................................................... 4-2b(3), 4-2, interference ................................................................4-3a(2) 4-3

..................................................................................... 4-3a(1) 4-3 Signal types:Role in successful operation ................................................4-1b, 4-1 Analog.............................................................................3-2b 3-2

.........................................................................................4-1c. 4-1 Digital ............................ .................................................3-2c 3-2Oscillator, harmonic order ............................................. 2-4c)(c) 2-14 Skywaves, propagation medium .......................................2-3c(1) 2-10Overdriving, effect ...................................................... 2-2c(1)(c) 2-4 Solar noise ..................................................................... 2-2b(3) 2-2Parasitic oscillation,...................................................................... Spurious emission, examples..................................... 2-2c(1)(b) 2-3

in control devices .....................................................2-4c(2)(b) 2-15 Spurious response interference ............................ 2-4c(1)(c) 2-14Power line interference: ................................................................ Squelch break, resulting from

Effect.................................................................... 4-3b(2)(b)4 4-7 interference .......................................................... 2-2c(1)(c)2 2-6Hum due to grounds............................................. 4-3b(2)(b)6 4-7 Substitution, to isolate interference....................................4-3a(2) 4-3Isolation of problems .............................................. 4-3c(3)(c) 4-14 Susceptibility characteristics:Origin ..................................................................... 2-2c(2)(a) 2-8 Frequency selective equipment...................................2-4c(1) 2-12Remedy........................................................................ 4-3d(4) 4-14 Non-frequency selective equipment ............................2-4c(2) 2-15

Radar signals: .............................................................................. Switch and relays, interference .................................... 2-2c(2)(e) 2-9As interference source ......... ..................................... 2-2c2)(g 2-9 Switching/patching, interferenceEffect ................................................................... 4-3b(12(b)2 4-6 problems ................................................................ 4-3c(3)(a) 4-13Remedy for interference............................................... 4-3d(2) 4-14 Test equipment:

Radiation: Frequency selective voltmeter, use ........................... 4-3a(2) 4-3Ground waves ..................... ............................................ 2-3c(1) 2-10 Oscilloscope, use ..................................................4-3b(1)(d) 4-4In ionosphere .................................................................. 2-3c(1), 2-10, Portable radio, use ....................................................4-3a(2), 4-3,

..................................................................................... 2-3c(2) 2-11 ..............................................................................4-3b(1)(d), 4-4,In troposphere .................................................................. 2-3c(1) 2-10 ............................................................................ 4-3c(2)(a)2, 4-11,Propagation ..........................................................................2-3c 2-10 .............................................................................. 4-3c(2)(b), 4-12,Skywave .......................................................................... 2-3c(1) 2-10 ............................................................................... 4-3c(3)(c) 4-14Receiver: ...................................................................................... Megger, use .......................................................... 4-3c(3)(b) 4-13Dual-conversion, interference....................................... 2-4c(1)(c) 2-14 Signal generator, use ......................................... 43b(1)(b)5, 4-4,

isolation of trouble .................................................... 4-3c(2)(a) 4-11 .............................................................................4-3b(1)(b)7 4-4Single-conversion, ........................................................................ Susceptibility to interference......................................... 2-4c(2)(d) 2-15

interference .............................................................. 2-4c(1)(c) 2-14 Thermal agitation, in equipment ...................................... 2-2b(1) 2-1Recognition procedures, for ......................................................... Time division multiplex (TDM)........................................ 3-2d(2) 3-5

interference ......................................................................4-3b 4-3 Transients, result ............................................................ 2-2c(2) 2-8Rectification, in control devices ....................................2-4c(2)(b) 2-15 Transmitter:Remedial procedures: .................................................................. Isolation of trouble.................................................. 4-3c(2)(b) 4-12

Antenna problems ...................................................... 4-3d(10) 4-15 Troposphere, propagation ................. ...............................2-3c(1) 2-10Cables, damaged ......................................................... 4-3d(5) 4-14 White noise interference ................ ............................. 4-3b(3)(a) 4-7Circuit component faults .............................................. 4-3d(6) 4-14Equipment grounding problems ................................... 4-3d(7) 4-14

Index 3

TM 11-490-5

By Order of the Secretary of the Army:

FRED C. WEYANDGeneral, United States Army

Official: Chief of StaffPAUL T. SMITHMajor General, United States ArmyThe Adjutant General

Distribution:Active Army

CNGB (2) USAINTA (5)TRADOC (10) USASCS (25)AMC (10) USARJ (5)CINCUSAREUR (25) MTMC (2)Armies (5) USCINCARRED (10)1st Sig Bde (10) Div (5)ASA (5) COE (2)HSC (2) ACS (5)USACIDC (5) USAEPG (2)5th Sig Comd (10) Sig Depots (2)6th Sig Comd (10) Sig FLDMS (2)7th Sig Comd (10) NORAD (2)31 ADA Bde (5) Corps (5)ARMCOM (5) USACC, (CC-OPS-OS) (25)MDW (2)

NG: State AG (2); Units: Same as Active Army except 1 copy to each unit.USAR: Same as Active Army except 1 copy to each unit.For explanation of abbreviations used, see AR 310-50.

U.S. GOVERNMENT PRINTING OFFICE: 1995-392-490

The Metric System and Equivalents

Linear Measure Liquid Measure

1 centiliter = 10 milliters = .34 fl. ounce1 centimeter = 10 millimeters = .39 inch 1 deciliter = 10 centiliters = 3.38 fl. ounces1 decimeter = 10 centimeters = 3.94 inches 1 liter = 10 deciliters = 33.81 fl. ounces1 meter = 10 decimeters = 39.37 inches 1 dekaliter = 10 liters = 2.64 gallons1 dekameter = 10 meters = 32.8 feet 1 hectoliter = 10 dekaliters = 26.42 gallons1 hectometer = 10 dekameters = 328.08 feet 1 kiloliter = 10 hectoliters = 264.18 gallons1 kilometer = 10 hectometers = 3,280.8 feet

Square MeasureWeights

1 sq. centimeter = 100 sq. millimeters = .155 sq. inch1 centigram = 10 milligrams = .15 grain 1 sq. decimeter = 100 sq. centimeters = 15.5 sq. inches1 decigram = 10 centigrams = 1.54 grains 1 sq. meter (centare) = 100 sq. decimeters = 10.76 sq. feet1 gram = 10 decigram = .035 ounce 1 sq. dekameter (are) = 100 sq. meters = 1,076.4 sq. feet1 decagram = 10 grams = .35 ounce 1 sq. hectometer (hectare) = 100 sq. dekameters = 2.47 acres1 hectogram = 10 decagrams = 3.52 ounces 1 sq. kilometer = 100 sq. hectometers = .386 sq. mile1 kilogram = 10 hectograms = 2.2 pounds1 quintal = 100 kilograms = 220.46 pounds Cubic Measure1 metric ton = 10 quintals = 1.1 short tons

1 cu. centimeter = 1000 cu. millimeters = .06 cu. inch1 cu. decimeter = 1000 cu. centimeters = 61.02 cu. inches1 cu. meter = 1000 cu. decimeters = 35.31 cu. feet

Approximate Conversion Factors

To change To Multiply by To change To Multiply by

inches centimeters 2.540 ounce-inches Newton-meters .007062feet meters .305 centimeters inches .394yards meters .914 meters feet 3.280miles kilometers 1.609 meters yards 1.094square inches square centimeters 6.451 kilometers miles .621square feet square meters .093 square centimeters square inches .155square yards square meters .836 square meters square feet 10.764square miles square kilometers 2.590 square meters square yards 1.196acres square hectometers .405 square kilometers square miles .386cubic feet cubic meters .028 square hectometers acres 2.471cubic yards cubic meters .765 cubic meters cubic feet 35.315fluid ounces milliliters 29,573 cubic meters cubic yards 1.308pints liters .473 milliliters fluid ounces .034quarts liters .946 liters pints 2.113gallons liters 3.785 liters quarts 1.057ounces grams 28.349 liters gallons .264pounds kilograms .454 grams ounces .035short tons metric tons .907 kilograms pounds 2.205pound-feet Newton-meters 1.356 metric tons short tons 1.102pound-inches Newton-meters .11296

Temperature (Exact)

°F Fahrenheit 5/9 (after Celsius °Ctemperature subtracting 32) temperature

PIN: 017706-000

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