BASIC PROPAGATION AND ANTENNAS - W3JJJ -- Historyw3jjj.com/downloads/AM 5-303 Basic Propagation and Antennas.pdf · AM 5 – 303 – Antenna Theory and Propagation 1-2 Ver. 1.0 1

AM 5-303

BASIC PROPAGATION

AND ANTENNAS

MAY 2012

DISTRIBUTION RESTRICTION: Approved for public release. Distribution is unlimited.

DEPARTMENT OF THE ARMY MILITARY AUXILIARY RADIO SYSTEM

FORT HUACHUCA ARIZONA 85613-7070

AM 5 – 303 – Antenna Theory and Propagation

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Table of Contents 1 INTRODUCTION ............................................................................................. 1-2 2 BASIC ANTENNA THEORY .................................................................................. 2-1

2.1 Basic HF radio propagation: ......................................................................... 2-1 2.2 Groundwave Propagation: ........................................................................... 2-2

2.2.1 Line of Sight (LOS) or Direct Wave: ........................................................... 2-2 2.2.2 Ground-Reflected Wave: ....................................................................... 2-3 2.2.3 Refracted Troposphereic Wave: ............................................................... 2-3

2.3 Skywave: ............................................................................................... 2-4 2.3.1 Ionospheric Layering: ........................................................................... 2-4 2.3.2 F layer: ........................................................................................... 2-4 2.3.3 E layer: ........................................................................................... 2-4 2.3.4 D layer: ........................................................................................... 2-4 2.3.5 Geometry of Ionospheric Refraction: ......................................................... 2-5

3 FREQUENCY SELECTION: .................................................................................. 3-1 3.1 Maximum Usable Frequency (MUF): ................................................................ 3-1 3.2 Lowest Useful Frequency (LUF): .................................................................... 3-1 3.3 Anomalous Ionospheric Propagation: .............................................................. 3-1

3.3.1 Sporadic E: ....................................................................................... 3-1 3.4 Ionospheric Storms: .................................................................................. 3-2 3.5 Polar Cap Absorption: ................................................................................ 3-2 3.6 Spread E/F: ............................................................................................ 3-2

4 MULTIPATH PROPAGATION: ............................................................................... 4-1 4.1 Doppler spread: ....................................................................................... 4-1 4.2 Noise: ................................................................................................... 4-5

4.2.1 Manmade Noise: ................................................................................. 4-5 4.2.2 Atmospheric Noise .............................................................................. 4-6 4.2.3 Galactic Noise ................................................................................... 4-6

5 DIVERSITY ................................................................................................... 5-1 5.1 Space Diversity: ....................................................................................... 5-1 5.2 Frequency diversity: ................................................................................. 5-1 5.3 Polarization Diversity: ............................................................................... 5-1 5.4 Time Diversity: ........................................................................................ 5-1 5.5 Path Diversity: ........................................................................................ 5-3 5.6 Diversity Combiners: ................................................................................. 5-3

6 PROPAGATION PREDICTION ............................................................................... 6-2 6.1 General: ................................................................................................ 6-2 6.2 Computer Prediction Models: ....................................................................... 6-3

6.2.1 PROPHET: ......................................................................................... 6-3 6.2.2 IONCAP 11-year Propagation Analysis: ....................................................... 6-4 6.2.3 IONCAP Point-to-Point Propagation Prediction Tables: .................................... 6-4 6.2.4 MINIMUF: ......................................................................................... 6-4

7 IONOSPHERIC SOUNDING .................................................................................. 7-2


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PREFACE Improvements Suggested corrections, or changes to this document, should be submitted through your State Director to the Regional Director. Any Changes will be made by the National documentation team.

Distribution Distribution is unlimited.

Versions The Versions are designated in the footer of each page if no version number is designated the version is considered to be 1.0 or the original issue. Documents may have pages with different versions designated; if so verify the versions on the “Change Page” at the beginning of each document.

References ALLIED COMMUNICATIONS PUBLICATIONS (ACP):

(1) ACP - 121 - Communications Instruction, General (2) ACP - 124 - Radiotelegraph Procedures (3) ACP - 125 - Radiotelephone Procedures (4) ACP - 126 - Communications Instructions – Radio Teletypewriter (5) ACP - 131 - Communications Instructions Operating Signals

US ARMY REGULATIONS AND MANUALS (1) AR 25-6 - Military Auxiliary Radio System (MARS) and Amateur Radio Program (2) FM 6-02.52 – Tactical Radio Operations (3) TM 5-811-3 - Electrical Design, Lightning and Static Electricity Protection (4) US Army MARS Net Plan (5) MIL-HDBK 413, DESIGN HANDBOOK FOR RADIO FREQUENCY COMMUNICATIONS SYSTEMS

DOD MANUALS (1) DOD Instruction 4650.2.

Other Government Documents (1) National Bureau of Standards - Monograph 80 (2) International Radio Consultative Committee (CCIR) -Report 322. (3) NTIA - Report 85-173

COMMERCIAL (1) Basic Electronics, Components, Devices and Circuits; ISBN 0-02-81860-X, By William P

Hand and Gerald Williams (2) Antenna Engineering Handbook, By Henry Jasik Editor, McGraw Hill

Contributors This document has been produced by the Army MARS Technical Writing Team under the authority of Army MARS HQ, Ft Huachuca, AZ. The following individuals are subject matter experts who made significant contributions to this document.

(1) William P Hand


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Acronyms and Abbreviations

Abbreviations Definition

AM Amplitude Modulated APC Allied Communications Publication AR Army Regulation CCIR International Radio Consultative Committee

CW Continuous Wave (Morse Code) dB Decibel

DoD Department of Defense F Fahrenheit FM 1. Frequency modulated

2. From 3. Field Manual

FOUO Official Use Only GHz Gigahertz 1,000,000 Hertz HF High Frequency Hz Hertz, one cycle per second IONCAP Ionospheric Communications Analysis and Prediction

Program ISD Intermediate and Short Distance Skywave KHz Kilohertz, (1x103 Hertz) (1,000 Hertz) Km Kilometers (1 thousand meters) L Length LOS Line of Sight LUF Lowest Useful Frequency MF Field Manual MHz Megahertz, (1x106 Hertz)(1,000,000 Hertz)

MIL Military MMF Multipath Minimization Factor MUF Maximum Usable Frequency Pf Picofarad (10 -12) RF Radio Frequency

SNR signal-to-noise ratio UHF Ultra High Frequency VHF Very High Frequency VLF Very Low Frequency


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1 INTRODUCTION An antenna is a device for radiating or intercepting electromagnetic wave energy. Virtually every transmitter and every receiver must have an antenna, and all antennas operate in accordance with certain basic principles, some of which are merely extensions of basic transmission line theory. Electromagnetic waves are produced whenever there is a radiofrequency current. Wires, RF coils, capacitors, and other components carrying this type of current are subject to RF power loss due to radiation. The amount of energy liberated depends upon the amount of current, the size and shape of the conducting materials, and the environment. In our development of antenna theory, we are assuming that the antenna is being used for transmission purposes. Most characteristics of a transmitting antenna will be found applicable to receiving antennas. This does not mean, however, that any antenna used for reception is suitable for transmission with maximum efficiency. Only under certain conditions are the two antennas interchangeable. The antenna is effectually a transformer coupling the signal to the atmosphere, and in turn, to the receiving antenna. There are about as many antenna types as there are people on this planet. In this discussion we will only consider the main general types and not get into the many very specialized types that have little valve for the type of radio communications we do.. There are generally 5 basic types of antennas but within each type there may be many designs, some for very specialized use that we will not consider. The basic types are:

1. VLF – Very Low Frequency 2. HF – High Frequency 3. VHF – Very High Frequency 4. UHF – Ultra High Frequency 5. Microwave

There are sub catagories within these broad types especially when you begin to consider microwave. We are only going to consider three of these general areas of antennas, HF, VHF and UHF. Microwave antennas are a study unto itself. Many oldtimers, like myself, will often in conversation mention Killocycles or Megacycles insted of Killohertz of Megahertz. That is an indication of someone being in radio a very long time. Another old term that us oldtimers used to use is”eather” when talking about the rf traveling through the “ether” insted of the ionisphere. I still sometimes use these terms along with some others when discussing radio with someone like using the term “condenser” insted of the modern word capacitor. Granted a capacitor does not condense anything but that was the term used for a capacitor even into the 1940’s so it comes out sometimes without a thought that the one I am talking to does not understand the term. The following paragraphs and equations were going to be part of the introduction to this manual, however I quickly realized that most members would not understand the math involved, so in many cases in this manual there is little or no math explaination to theory introduced.


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Most antennas reversibly link radiation fields to currents flowing in wires at frequencies ranging from sub-audio through the far-infrared region. Aperture antennas that link radiation fields to materials can operate in microwave, infrared, visible, ultraviolet, X-ray, gamma ray, and even higher energy regimes. The design of lens and mirror systems for coupling radiation directly to materials is generally called “optics”, and the use of these optical techniques for coupling radiation to wires or waveguides is often called “quasi-optics”. For the most part, all of these systems can be characterized by the definitions that follow. A transmitter with available power PT will radiate PTR , where the antenna radiation efficiency :

and typically is close to unity for most antennas. Ideally, this power would be radiated exclusively in the intended direction, but in practice some fraction is radiated instead to side into sidelobes, or rearward 2π steradians in the form of backlobes. The ability of an antenna to radiate energy in a desired direction is characterized by its antenna directivity, , which is the ratio of power actually transmitted in a particular direction to that which would be transmitted had the power PTR been radiated isotropically; therefore, directivity is sometimes called “directivity over isotropic”. Directivity can be defined as:

where P(f,Θ,Ф) (watts/steradian) integrated over 4π steradians equals PTR . ____________

As can be seen from the above, antennas can be complex. In this document we will attempt to keep such math and advanced theory to a minimum. It was decided to place basic descriptions only in this document with minimal deep theory and math design data (equations) for a specific antenna will be given in AM 5-304. We will no attempt to give design data in this document.


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2 BASIC ANTENNA THEORY There are two main types of propagation "groundwave" and "skywave". Groundwave propagation occurs when the receiving station is sufficiently close to the transmitting station, and is able to receive the portion of the transmitting station's signal which clings to the ground. The range of groundwave propagation varies with the type of antenna at the transmitting station, characteristics of ground between stations, among other factors. It can be anywhere from a few miles, to a few dozen miles. Distances beyond the range of the groundwave signal are covered by skywaves. Skywaves are waves radiated upward at some angle from the antenna, and (we hope) are reflected from the ionosphere, to return to earth further away. The ionosphere is a high altitude region of Earth's atmosphere composed of gaseous atoms broken into ions. The sun is the source of ionizing energy, so the condition of the ionosphere varies with time of day, season, 11-year sunspot cycle, and 27-day rotation of the sun. Layers of atmosphere effecting radio propagation are the D, E, and F layers. I won't go into much detail in outlining their roles. If you're interested in this topic, entire books have been devoted to it. In a nutshell, it's the F layer which is usually involved in reflecting our signals back to earth, while the D layer absorbs our signals. The E-layer can either help, or hinder. Long distance propagation of radio waves is usually achieved by their being reflected from the ionosphere, and returning to earth some distance away from their point or origin. Radio waves radiated at a very low angle of radiation travel a long way before finally making it up to the ionosphere, and strike it at a very shallow angle and return to earth far away from their point of origin. As the angle of radiation goes up, radio waves strike the atmosphere at a more moderate angle, and return to earth closer to their point of origin. For any given frequency and current state of the ionosphere, there may be some maximum angle of incidence at which the ionosphere will reflect signals back to earth. Signals which strike the ionosphere at a higher angle of incidence than the current maximum will not be reflected at all, but will continue on out into space. The area of earth where reflection would have occurred will be in what we call the "skip zone" (unless it's close enough to the signal source to receive the groundwave signal). The skip zone is the region consisting of areas of earth's surface outside the radius the transmitting station's groundwave will reach, and yet not far enough away to receive reflections of skywaves.

2.1 BASIC HF RADIO PROPAGATION:

Radio wave propagation at HF is characterized by two basic methods: groundwaves and skywaves and within each there are numerous variations. The groundwave enables short-range communications. The skywave enables global communications. Groundwave radio waves travel on or near the surface of the earth whereas a skywave depends on radio wave refraction in the ionosphere. (Some texts reference the ionospheric propagation reflection from the ionosphere.)


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2.2 GROUNDWAVE PROPAGATION:

Groundwaves have three distinct components: Surface wave, direct wave, and ground-reflected wave. A fourth component, a troposphere-refracted wave sometimes occurs in conjunction with air density gradients. This is illustrated in Figure 2-1. The surface wave component moves along the surface of the earth. Therefore, conductivity of the surface is a primary factor in attenuation of any surface wave signal. Energy is absorbed by the earth in accordance with its conductivity. Absorption increases with frequency and limits useful surface-wave propagation to lower HF ranges. Vertically polarized waves propagate better over earth’s surface than horizontally polarized because they incur lower attenuation. (Polarization of radio waves is defined by the origin of electric lines of force in the electromagnetic field relative to the surface of the earth. The polarization with which radio waves are “launched” is determined by the transmitting antenna.) The field intensity of groundwaves depends upon radio frequency, transmitter power, transmitting antenna characteristics, and electrical characteristics (conductivity and dielectric constant) of the terrain. Low and very low frequencies are propagated much better by surface wave than are high frequencies. When high-powered transmitters and efficient antennas are used, surface wave has a maximum range of about 500 km (300 mi) at 2 MHz and decreases with increasing frequency. About 80 km (50 mi) represents the usual maximum range. The range of groundwave from commonly used transportable equipment is less.

Figure 2-1

Groundwave Components

2.2.1 Line of Sight (LOS) or Direct Wave:

Direct wave follows a direct path through the troposphere from transmitting antenna to receiving antenna. This path is slightly curved by normal refraction by the atmosphere, enabling propagation beyond the visible horizon.


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2.2.2 Ground-Reflected Wave:

A portion of the propagated wave is reflected from earth’s surface at some point between the transmitting and receiving antenna. Reflection causes a phase change of transmitted signal. The phase change results in a reduction, or enhancement of the combined received signal, depending on time of arrival of reflected signal relative to other components. Relative times of arrival are, in turn, dependent upon the relative path distances traveled by the wave components.

2.2.3 Refracted Troposphereic Wave:

The refracted troposphereic wave (not to be confused with normally refracted direct wave results when abrupt differences in atmospheric density and refractive index exist between large air masses. This type of refraction, associated with weather fronts, is normally not significant at Frequency dependence of groundwave propagation. At frequencies below about 5 MHz, surface waves are favored because the ground behaves as a conductor for electrometric energy. Above 10 MHz, the ground behaves as a dielectric. Below 10 MHz, path ground conductivity is a critical factor. Generally surface waves travel best over sea water. Table I provides a rough sentiments of relative conductivity of various surface types. As frequencies approach 30 MHz, losses suffered by surface wave become excessive and transmission is possible only by means of direct waves.

TABLE I. Conductivity of Various Types of Surface

Type of Surface

Relative Conductivity

Sea water Good

Fresh water Fair

Soil Fair

Rocks Poor

Desert Poor

Jungle Bad

Artic Bad


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2.3 SKYWAVE:

Since skywave propagation results from radio wave refraction in the ionosphere, this mechanism is generally referred to as ionospheric radio propagation. The ionosphere is where the atmosphere where ionization takes place with sufficient intensity to produce a significant ion density gradient with altitude. The density gradient is manifested as a gradual change in radio refractive index, which causes radio waves at HF to be refracted. The ionosphere is divided into layers as shown in Figure 2-2.

2.3.1 Ionospheric Layering:

Solar radiation (ultraviolet light and x-rays) and, to a lesser extent, cosmic rays impinge on ionospheric gases and cause ionization. Since these ionization sources vary in energy level, they penetrate to different depths of the atmosphere before causing ionization. The natural grouping of energy levels results in distinct layers being formed at different altitudes. Figure 3 illustrates ionospheric layering and gives the generally accepted layer designations. Not shown is the density gradient within the layers. Density increases with altitude to a maximum value, then decreases or remains constant up to the next higher layer, if any.

2.3.2 F layer:

The highest ionized region is called the F layer. It ranges in height from 130 to 460 km and is the primary medium for long distance HF propagation. If sporadic ionospheric disturbances are ignored, the height and density of this region varies in a predictable manner diurnally, seasonally, and with the 11-year sunspot cycle. Under normal conditions it exists 24 hours a day. At night, the layer has a single density peak and is called, simply, the Flayer. During the day, the absorption of solar energy results in the formation of two distinct density peaks. The higher and most important of the two is called the F2 layer. It ranges in height from 200 to 460 km. The lower peak, known as the FI layer, ranges in height from 130 to 300 km and seldom is predominant in supporting HF radio propagation. During daytime, the F layers increase in density, reaching a maximum several hours after local noon. Then they decrease exponentially from the maximum, reaching a combined minimum during the night.

2.3.3 E layer:

The lowest layer to support HF radio propagation is the E layer. The average altitude of its central region is about 110 km. The atmosphere at this height is dense enough to allow rapid deionization as solar energy ceases to reach it. Ionization of this layer commences near sunrise and ceases shortly after sundown. Irregular cloud-like layers of ionization, called sporadic E, often occur in the region of normal E-layer appearance. These areas are highly ionized and are sometimes capable of supporting propagation of frequencies within and well above the HF range.

2.3.4 D layer:

During daylight hours, there also exists a weakly ionized region at 50 to 90 km altitude, known as the D layer. The density of this region is closely proportional to the solar elevation angle. Due to the greater penetration ability of higher radio frequencies, the D layer has little effect on frequencies above about 10 MHz. At lower frequencies, however, absorption by the D layer is significant.


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Figure 2-2

Atmospheric Layers

2.3.5 Geometry of Ionospheric Refraction:

As the path traversed by radio waves of a particular frequency, f, enters an ionospheric layer at a

sufficiently oblique angle, o, relative to a line extended from the center of the earth to entry point, refraction gradually curves the path and turns it back toward the surface of the earth. Refraction is gradual because ion density and, hence, refractive index of each ionospheric layer increases with height, up to its maximum. This gradual refraction can be viewed as equivalent to a reflection from an abrupt discontinuity in refractive index, at a height referred to as the virtual height. This is illustrated in Figure 2-3.


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The highest frequency reflected at vertical incidence is called the critical frequency, fc. Through an application of Snell’s Law (reference National Bureau of Standards Monograph 80)’ the highest frequency, f returned to earth from oblique incidence is given by:

f = fc sec Θ In the equation, Θ (Theta) is the angle of incidence, measured from normal to the plane of the layer. This relationship, known as ‘secant law’, shows that at oblique incidence, the ionosphere can reflect much higher frequencies than at vertical (normal) incidence. It can also be seen that the greater angles of incidence, the higher the frequency that will be returned to earth. (The secant law, as stated, applies to a plane ionospheric layer. For a treatment of the curved ionosphere, reference National Bureau of Standards Monograph 80.)

c

FIGURE 2-3 Concept of Virtual Height


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3 FREQUENCY SELECTION:

3.1 MAXIMUM USABLE FREQUENCY (MUF):

The highest frequency at which radio waves are returned to earth is known as the maximum frequency that can be used to effect ionospheric propagation between two points at a specific time. This frequency is termed the Maximum Usable Frequency (MUF). Because ionization of the ionospheric layers is extremely variable, MUF must be statistically defined. The accepted working definition of MUF is the highest frequency predicted to occur via a normal reflection from the F2 layer on 50 % of the days of the month at a given time of day on a specified path.

3.2 LOWEST USEFUL FREQUENCY (LUF):

The Lowest Useful Frequency (LUF) is the statistically calculated lowest frequency where the field intensity at the receiving antenna is sufficient to provide required signal-to-noise ratio (SNR) on 90 % of undisturbed. Unlike MUF, LUF is partly dependent on transmitter power output and on noise levels at the receiving station.

3.3 ANOMALOUS IONOSPHERIC PROPAGATION:

Deviations from normal ionospheric propagation occur as the result of certain irregularities and transient conditions in the ionosphere. The most significant anomalous propagation mechanisms are sporadic E (Es), sudden ionospheric disturbances (SIDs) , ionospheric storms, polar cap absorption, and spread-F or spread-E, (depending upon the ionospheric layer affected).

3.3.1 Sporadic E:

In addition to relatively regular ionospheric layers (D, E, and F), there arc a number of irregular layers of transient occurrence. The significant irregular reflective layer is sporadic E (Es) since it occurs in the same altitude region as regular E. It is theorized that E, occurs as a result of ionization from high altitude wind shear in the presence of the magnetic field of the earth, rather than from ionization by solar and cosmic radiation. Areas of Es generally last only a few hours, and move about rapidly under the influence of high altitude wind patterns. When Es occurs, it produces a marked effect on radio propagation paths which normally involve the regular layers (Reference Figure 2-2). Although Es is difficult to predict, it can be used to advantage when its presence is known. Close to the equator, Es occurs primarily during daylight hours with little seasonal variation. In the aurora zone, by contrast, it is most prevalent during the night and again shows little seasonal variation. In middle latitudes, Es occurrence is subject to both seasonal and diurnal variations. In these latitudes, it is more prevalent in local summer than in winter and during daylight than at night.

3.3.1.1 Sudden Ionospheric Disturbances (SIDs):

A (SID) can totally disrupt HF propagation and occur without warning and can last from a few minutes to several hours. All radio links operating within, or even partially in, the daylight side of the earth will be affected. SIDs are normally associated with solar flares; therefore, can be related to the 11-year sunspot cycle. The primary cause of propagation disruption (fadeout or blackout) is a sudden, abnormal increase in ionization of the D region. The region then absorbs all lower range of HF frequencies and partially absorbs higher frequencies.


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Figure 3-2

Anomalous Preparation by Sporadic E

3.4 IONOSPHERIC STORMS:

Ionospheric storms result from large increases in solar activity. This activity can produce large variations in critical frequencies, layer heights, and absorption. These storms may last from several hours to several days. Intensity varies and effects usually extend over the entire earth, Solar storms usually follow solar-flare-initiated SIDs by anywhere from 15 minutes to 72 hours. Ionospheric storms also occur during periods of very high sunspot activity without SIDs. During storms, i\Ionospheric Propagation is characterized by low received signal strengths and flutter-fading, a form of fading that is especially deleterious to voice communications. During the first few hours of a severe ionospheric storm, the ionosphere is in a state of turbulence, layer-forming stratification is destroyed, and propagation is, consequently, erratic. During the later stages of severe storms, and throughout more moderate storms, the upper part of the ionosphere is expanded and diffused. As a result, the virtual heights of layers are much greater arid the maximum usable frequencies much lower. Absorption of radio waves in the D layer is also increased.

3.5 POLAR CAP ABSORPTION:

When high energy protons are deflected toward Polar Regions by the magnetic field of earth, D-layer ionization in the area is increased. Severe absorption occurs to radio waves traversing the Polar Regions. Resultant attenuation of signals may last for several days, especially when the polar cap is in daylight.

3.6 SPREAD E/F:

Irregularities in ionospheric surfaces scatter, or defocus, radio waves. When this occurs in the F layer, the disturbance is called spread-F; in the E layer, spread-E. Surface abnormalities occur as a result of random motion within ionized layers and changes in ion density profiles.


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4 MULTIPATH PROPAGATION: Most HF antennas radiate over a broad vertical angle. As a result, the radiated energy impinges on one or more ionospheric layers at different angles of incidence. The refracted waves follow different paths. Some paths may undergo reflection from the ground and return to the ionosphere one or more times before reaching the destination. Such multihop paths, being physically longer, delay the waves traversing them substantially relative to a single-hop path. Consequently, the received signal is dispersed over time. This is particularly deleterious to digital transmissions at high data rates, since it causes intersymbol interference. The multipath propagation mechanism is illustrated in Figure 4-1. The difference in time of arrival of waves traversing different paths is called multipath spread. Up to a point, multipath spread is inversely related to distance, because of the smaller number of modes that propagate or reflect on longer paths. This relationship is plotted in Figure 4-2. Multipath spread can be minimized by selecting frequencies as close to the MUF as possible. Experimental data has shown that as the MUF is approached, the time dispersion decreases. Curves based on the experimental data are given in Figure 4-2. The ordinate of the graph is labeled multipath minimization factor (MMF). The scale is the ratio of the selected frequency to the MUF. Thus, the MTSF defines a frequency above which the multipath spread will be less than that determined by the curve.

4.1 DOPPLER SPREAD:

Variations in refractive index cause volumes of different ion densities to move with respect to each other. Variations occur in both horizontal and vertical planes caused by variations in density of ionization and molecules in adjacent regions and by variations in the magnitude of geomagnetic fields. The relative motions between density volumes produce Doppler shifts of about 1 hertz (Hz). Such shifts transform single-frequency signals into a spectrum of frequencies centered on a mean frequency. The result is evidenced as radio signal fading. Doppler spread has little impact on analog signals but can cause severe problems in data transmissions, particularly at higher data rates. Although Doppler spread does not lend itself to detailed analysis, it has been empirically deter-mined that it establishes a fundamental error probability in data transmission over a single path. It is known, however, that Doppler shift occurs independently on different paths. For this reason, path diversity minimizes the effect. This is illustrated in Figure 4-1, showing the probability of error, P, for no diversity and for dual diversity as a function of Doppler spread for a differential quadriphase phase-shift keying signal.


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FIGURE 4-1 Multipath Spread as a Function of Distance


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Fig

ure

4-2

Exam

ple

of

Mult

ipath

Recepti

on


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Figure 4-3 Multipath Spread as a Function of Distance


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4.2 NOISE:

Noise is the limiting factor in transmitting information by radio signals. In HF systems, three sources of noise predominate: man-made, atmospheric, and galactic.

4.2.1 Manmade Noise:

Near civilization, man-made noise is the predominant noise component over much of the HF band. It emanates from power lines, industrial machinery, and electrical devices such as relays, voltage regulators, arc-welders, and ignition systems of internal combustion machines. It occurs most strongly in cities and industrial areas. It tends to be cyclic, corresponding to period of noise-generating devices; it also tends to vary during the day in accordance with working periods. Man-made noise is less strong in residential areas, and even less significant in rural areas. I n areas remote from civilization, it ceases to be a factor. Figure 4-1 shows the relationship of noise to frequency for the different types of areas. In dense electromagnetic environments of man-made noise, such as may be experienced on chips, the man-made noise levels may exceed those shown for business areas in Figure 4-4.

Figure 4-4

Area Noise Comparison Deliberate man-made noise known as jamming can prevent information transmission by conventional HF systems. Such noise can be generated by an enemy and transmitted on the operating frequency in the direction of the receiving terminal. Specially designed HF radio systems are required for operation in the presence of severe jamming.


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4.2.2 Atmospheric Noise

Atmospheric noise comes primarily from lightning discharges all over Earth. Noise from local lightning appears as impulse noise. Atmospheric radio noise is characterized by large, rapid fluctuations, but if averaged over several minutes, the average values are nearly constant during a given hour with variations rarely exceeding f2 dB except during local thunderstorms or at sunrise-sunset. The amplitude of noise varies approximately inversely with the square of the frequency. It is propagated in all directions similar to normal radio signals. In general, atmospheric noise is greater at night for frequencies from 1 to 5 MHz, because of long-range nighttime propagation of those frequencies. From 10 to 15 MHz, there is little difference in intensity between day and night. Moreover, atmospheric noise level is greater in local summer than winter, as may be expected from relative frequency of thunderstorms. Atmospheric noise becomes progressively less important at the higher latitudes of the temperate zones, because of both distance from equatorial sources and aurora absorption of propagated waves. Charts showing expected values of atmospheric radio noise worldwide are available all year and for 4-hour increments of any typical day during a season. Charts are published in Report 322 of the International Radio Consultative Committee (CCIR). NTIA Report 85-173 has updated some of these charts.

4.2.3 Galactic Noise

Galactic noise, also known as cosmic noise, originates in outer space. Its characteristics are similar to thermal, or white, noise. Sources of cosmic noise are distributed unevenly throughout space. They tend to be concentrated in several regions. The principal region is near the center of the Milky Way galaxy. Consequently, for reception on a directional antenna, galactic noise varies from hour to hour and day to day, as the antenna beam intercepts the galactic plane. Galactic noise is relatively insignificant, compared to atmospheric noise, below 20 MHz. From 20 to 30 MHz, galactic noise constitutes a minor noise component at sites having low man-made noise.


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5 DIVERSITY Diversity reception is a means of counteracting the signal fading that occurs over ionospheric radio paths. Diversity reception involves two or more receivers. Diversity combiners then combine the signals from these receivers. Signal enhancement can be accomplished by diversity operation. It is attributable to different radio paths undergo largely uncorrelated variations. That is, variations such as amplitude fading do not occur simultaneously to an identical extent in two separate paths. The degree of improvement by using diversity operation depends on the lack of correlation between propagation-induced variations in signals received over two or more paths. The common forms of diversity are space, frequency, polarization, time, and path.

5.1 SPACE DIVERSITY:

Space diversity requires two antennas, each feeding a different receiver, spaced sufficiently far apart such that the perturbations in the signals arriving at each are largely uncorrelated. Generally, a spacing of five wavelengths provides reasonable lack of correlation. Of course, wider spacing reduces the correlation even more, but the rate of improvement is usually too small to justify the increased land area and transmission line runs required. The signals at the receiver outputs are routed to a diversity combiner. The effect is to fill in the deep fades and, over a period of time, the combined signal has a higher average value than either of the separate signals alone.

5.2 FREQUENCY DIVERSITY:

Frequency diversity is based on the assumption that fading does not affect all frequencies equally. It may be implemented by using two carrier frequencies or two tones on a single carrier. Both methods are wasteful of the already congested HF spectrum. Carrier-frequency diversity, which provides superior performance to the other method, requires two separate transmitters and two separate receivers, but no additional antennas if diplexers and multicouplers are used. Normality, ground-based frequency diversity installations will employ separate antennas for each transmitter.

5.3 POLARIZATION DIVERSITY:

Where space is not available to adequately separate two large antennas, polarization diversity provides a compromise solution. By using two receiving antennas - one horizontally polarized; the other, vertically polarized - which can be placed in close proximity to each other, the desired signal can be received over two somewhat uncorrelated paths. Each antenna feeds a separate receiver. While this type of diversity is effective on both medium and long paths, space diversity with adequate separation provides better performance on long paths.

5.4 TIME DIVERSITY:

Time diversity, used in data communications, takes advantage of the fact that error-producing perturbations usually occur in bursts of fairly well defined duration. Time diversity technique places the same information on the same medium two or more times, separated by a period longer than the normal duration of an error-producing perturbation. Although this form of diversity requires no additional antennas, transmitters, or receivers, it does require modems capable of repeating message units at the proper intervals and combining the received message units in the proper time sequence. The method provides important signal enhancement even with frequent error bursts, but it does so at the expense of reduced data throughput.


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5.5 PATH DIVERSITY:

Path diversity uses geographically dispersed paths, i.e., separate HF radio links, to carry identical signals simultaneously. While it is expensive in terms of HF radio facilities, it provides substantial improvement, particularly in view of the fact that each of the paths can employ its own independent diversity for further improvement.

5.6 DIVERSITY COMBINERS:

There are two categories of diversity combiners, depending on the stage at which combining takes place: pre-detection and post-detection.

1. Pre-detection combining takes place at the IF stage (or the RF stage in some systems) 2. Post-detection, at baseband (after demodulation). In VFCT operation, post-detection

combining takes place after the VFCT modem. Three types of diversity combiners commonly used are selection, equal gain (or linear adder), and maximal ratio (or ratio squared), selection combiner simply selects one receiver at a time. Full diversity improvement of this type occurs only when the receiver with the strongest signal is selected. In actual operation, the receiver with the strongest signal-plus-noise is selected. The equal gain combiner simply adds receiver outputs, providing somewhat more improvement than the selector type. The maximal ratio combiner adjusts the gain of the combined signal in accordance with the ratio of the two signals. It provides the best performance of the three types and works well as a pre-detection combiner.


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6 PROPAGATION PREDICTION

6.1 GENERAL:

Department of Commerce, available through USAISESA, cover groundwave and skywave propagation in three sets of books. The skywave books are published in two sets: one for intermediate and short distance skywave, the other for air/ground skywave. Also, the DoD Electromagnetic Compatibility

Analysis Center (ECAC), Annapolis, MD, publishes quarterly propagation supplements for the U.S. military services.

1. Groundwave propagation charts for selected areas of the world provide predicted distance

ranges. Charts are currently available for 24 areas located in the continental United States, Central America, Europe, Africa, and parts of Asia, South America, and Greenland. Predictions in the charts cover all seasons.

2. The Intermediate and Short Distance Skywave (ISD) give the MUF, FOT, and LUF. They are

useful for propagation predictions over paths up to 2400 km in length. There are 33 ISD books covering the continental United States, Central America, South America, Europe, Africa, Greenland, and parts of Asia. The air/ground skywave (A/G) books give lists of frequencies for air-ground communications within an 8340 km radius of 57 selected ground stations throughout the world.

3. In addition to groundwave and skywave, propagation books, USAISESA issues reports in the

form of messages on day-to-day ionospheric activity. The reports cover 7-day periods. They are based on messages and alerts from Global Weather Central, an element of the Air Weather Service at Offutt Air Force Base, Nebraska. The reports review propagation conditions for the prior week and forecast solar and geomagnetic activity for the next 7 days (short-term forecast) and the next 30 days (long-term forecast). The reports include narrative descriptions of propagation conditions over polar, auroral, and low, mid, and equatorial latitudes.


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6.2 COMPUTER PREDICTION MODELS:

Computer prediction models serve a number of purposes, ranging from analysis of proposed communications links to electromagnetic compatibility (EMC) assessment and the effects of nuclear blasts. Table 6-1 is a list of some of the programs available. A number of computer prediction

models and computerized prediction services are available through USAISESA; among them

PROPHET, Ionospheric Communications Analysis and Prediction Program (IONCAP), MINIMUF and MINIMUF BASIC. Predictions are also available from the Air Force Communications Command (AFCC) Naval Electronics Systems Command (NAVELEX), and ECAC. The National Telecommunications and Information Administration Institute for Telecommunication Sciences also has prediction programs and other data at: http://www.its.bldrdoc.gov/programs/its-technical-programs.aspx.

TABLE IV Computer Prediction models for HF

Model Acronym Agency

High frequency communications assessment model HFCAM ECAC

HF electromagnetic compatibility HF EMC2 NOSC

HF maximum usable frequency evaluation HFMUFES-4 ITS

Ionospheric comm. analysis and prediction program IONCAP ITS

Minicomputer model for predicting MUF in HF comm. MINIMUF-3.5 NOSC

Effect of nuclear burst on HF communications NUCOM-II SRI

Propagation forecasting and assessment system PROPHET NOSC

Quiet-time lowest usable frequency QLOF NOSC

HFMUFES -4 ionospheric propagation model RADARC NRL

Sudden ionospheric disturbance grid SIDGRID NOSC

HF skywave propagation model SKYWAVE ITS

X-ray flare and shortwave fade duration model XRAY FLARE NOSC

ECAC Electromagnetic Compatibility Analysis Center ITS Institute for Telecommunications Sciences NOSC Naval Ocean Systems Command NRL Naval Research Laboratory SRI Stanford Research Institute

6.2.1 PROPHET:

PROPHET is a group of HF computer models designed for field use. These programs define the propagation constraints and provide data for propagation planning. This data is intended to improve HF link effectiveness, assist in achieving a low probability of intercept and location, and to reduce vulnerability to jamming. All models in this group are designed for programmable calculators. The models require ongoing software and engineering support which is provided by the Army Master Prediction Support System (AMAPSS), a USAISESA system.


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6.2.2 IONCAP 11-year Propagation Analysis:

IONCAP was developed by Institute for Telecornmunications Sciences (ITS), National Telecommunications and Information Administration (NTIA) , U.S. Dept. of Commerce. It includes an 11-year propagation analysis, to provide data for HF system design for specified level of performance over extremes of sunspot activity experienced in the solar cycle. Program users input transmitter and receiver locations, transmitter power output, required signal-to-noise ratio, frequency limitations, and estimated man-made noise levels. The program contains the necessary ionospheric physics data. The program can assume either isotropic (theoretical point source) antennas or models of antennas from either internal or external sources. Special output routines can be invoked to provide required frequency range, range of required take-off angles, required overall system gain, and low-end cutoff frequencies. Data is intended to provide system designs that will yield 90 percent circuit reliability for 90 percent of the hours over the 11-year solar cycle.

6.2.3 IONCAP Point-to-Point Propagation Prediction Tables:

Program users input transmitting and receiving location coordinates antenna types, required signal-to-noise ratio, desired month, sunspot number, man-made noise level, and the available frequency complement. The program produces a table of reliabilities as a function of frequency and time. The reliabilities are presented to indicate the percentage of undisturbed days during a month that the required signal-to-noise ratio is met or exceeded. Charts are available on a one-time basis or as a recurring monthly or quarterly report.

6.2.4 MINIMUF:

MINIMUF is a PROPHET program for predicting MUF using small-scale microcomputers. The original version was developed by Naval Ocean Systems Center (NOSC). It has only 80 program steps and permits use of only a few input variables: transmitter and receiver location, time, date, and sunspot number (SSN). It assesses propagation by way of the F layer only with a linear MUF/SSN relationship. It is reasonably accurate for frequencies between 2 and 30 MHz and distances from 800 km to 8000 km. To extend MINMMUF usefulness, it is available in BASIC. Later versions, though longer and more complex, include such accuracy improving routines as a nonlinear MUF/SSN relationship.


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7 IONOSPHERIC SOUNDING Sounding of the ionosphere provides research data for the various propagation forecast publications and data for frequency selection at the time of the sounding. A sounder is, in effect, an HF test link. It consists of a transmitter and receiver and uses its own or the operating system antennas. A step-sounder transmitter emits signals at discrete frequencies over a range of frequencies. Chirp sounders(R) sweep the frequency range. Time delay between transmission and reception on each frequency is translated into ionospheric layer height. An oscilloscope-type display, known as an ionogram, shows height as a function of frequency, thus, giving a display of available skywave transmission modes. The groundwave return is also displayed. Some modern sounding equipment has added a printout capability to the display. There are two basic classifications of ionospheric sounding, depending on the location of the sounding equipment with respect to the ionosphere: bottom side and topside. Bottom side sounding is accomplished from stations below the ionosphere, generally ground stations. Topside sounding is accomplished from satellites above the ionosphere. Both types use either vertically incident or obliquely incident radio waves. Vertical-incidence sounders use collocated sounding transmitters and receivers. They direct the signals vertically to the ionosphere; an algorithm is used to convert vertical incidence data for use on oblique paths. Oblique sounders, using a transmitter at one end of a circuit and a receiver at the other end, provide more accurate results for the usual oblique radio path.