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Microwave Communications
Microwaves
Signals with a frequency greater than 1 GHz.The microwave region is generally
considered to extend to 300 GHz.Point-to-point communications.Utilize the line of sight path, which means
the two antennas (for transmitter and receiver) should see each other (no obstructions).
Band Frequency (GHz) ApplicationL 1 – 2S 2 – 4 Marine radarC 4 – 8 Commercial use,
satellitesX 8 – 12 MilitaryKu 12 – 18 Commercial use,
satellitesK 18 – 27 Commercial use,
satellitesKa 27 – 40 MilitaryU 60 – 80W 80 – 100
Microwave Radio-Frequency Assignments
Types of Microwave Paths
Line of Sight (LOS) Path
Grazing Path Obstructed Path
No obstruction exists and antennas could see each other.
Line of Sight (LOS) Path
The microwave beam barely touches the obstruction; zero clearance.
Grazing Path
The microwave beam is hindered by an obstruction.
Obstructed Path
Factors Affecting Microwave Energy
Fading Refraction Absorption Diffraction Attenuation Reflection Ducting and Thermal Inversion Earth Búlge
Variation of field strength caused by changes in transmission medium.
Fading
Change in direction due to changes in transmission densities, temperature, pressure, water vapor.
Refraction
Energy loss due to absorption of wave by atmospheric elements such as rain, snow, oxygen, clouds and vapors.
Absorption
The change in propagation direction of waves due differences in density / velocity of medium.
Diffraction
A decrease in intensity of energy to spreading of energy, transmission line losses or path losses between two antennas.
Attenuation
Occur when waves strike smooth surfaces.
Reflection
Trapped waves bounce back and forth in a duct caused by temperature and humidity inversion.
Ducting and Thermal Inversion
Earth’s curvature presents LOS obstruction and must be compensated using 4/3 earth radius for atmospheric bending of waves.
Earth Búlge
1. Large information handling capacity (256 – 9600 kbps)
2. High reliability through diversity techniques.3. Lower power consumption4. Carry wideband circuits for high speed data;
high quality voice channels.5. Could be fitted with anti-jam equipment,
adaptive modems and other accessories.6. Forward error correction and hitless switching.7. Microprocessor controlled pre-detection
combing.
Advantages of Microwave Communications
The earth, being spherical, limits the distance of which of line of sight in possible. The parameter which considers wave bending on the earth’s curvature is the K-factor.
Earth Curvature on RF Propagation
Unitless value which is the ratio of a hypothetical effective earth radius over 6370km, which is the true mean earth radius.
K-Factor
K = r / ro
Under his condition, the radius of the earth (fictitious radius), appears to the microwave beams to be longer than the true radius; that is, the earth appears flatter because of the tendency of the beam to refract downward in the atmosphere and follow the earth.
Translation of Various K-FactorsStandard Condition
K = 4 / 3 normal condition of the atmosphere.
K = bigger than 4 / 3 (abnormal condition)
Translation of Various K-FactorsSuper-Standard Condition (Super-Refraction)
When this condition results in an effective flattening of the equivalent earth’s curvature.
(When K = infinity, it is flat)
K = smaller than 4 / 3 (abnormal condition)
Translation of Various K-FactorsSub-Standard Condition
Typical microwave links are based on a K-Factor of 4 / 3. Other K-Factor values are used with the conditions of the link are known to be serve or difficult to propagate over.
When K = 1 / 2 the unusual refill condition is also called “earth bulging”.
Shows the cross-section” of the earth’s surface where the radio path passes over.
Path Profiling
Determines the actual clearance along the path, antenna heights and overall reliability. Normally scaled at 4, 2, or 1 mile inch on the horizontal and 25, 100 and 400 feet on the vertical.
Radio Path Profile
Graph showing contour lines, thereby, elevations and distances between two points are known.
Topographical Maps
Microwave Transmission Calculations: Path Calculations / Link Budget
(hT / d1) – (d1 / 2) = (hR / d2) – (d2 / 2)
1. Consider the following for K-Factor of 4/3:
Height of a microwave system
hT(ft) = (d1(mi) x d2(mi)) / 2
2. Transmitter Output (dB)
PT(dBm) = 10log (PT / 1mW)
PT(dBμ ) = 10log (PT / 1μW)
PT(dBW) = 10log (PT / 1W)
3. Waveguide Loss (WL)
WL = (dB / m, ft) x m, ft
4. Gain of Parabolic Antenna
English system:
GdB = 7.5 + 20logf GHz + 20logB ft
Metric system:
GdB = 17.8 + 20logf GHz + 20logB m
5. Effective Radiated Power (ERP)
ERP = PT – WL + G
6. Free Space Loss (FSL)
English system:
FSL dB = 96.6 + 20logf GHz + 20logD miles
Metric system:
FSL dB = 92.4 + 20logf GHz + 20logD km
7. Net Path Loss (NPL)
NPL dB = Total Losses – Total Gains
8. Received Signal Level (RSL)
RSL dB = PTdBm – NPLdB
RSL=Transmitter Output – Waveguide Loss (Tx) +Antenna Gain (Tx) – FSL + Antenna Gain (Rx) – Waveguide Loss (Rx)
9. Noise (or Detection, or Absolute) Threshold (NT)
NT dBm = - 114 + 10logBWMHz + FdB
Sensitivity Threshold of a Receiver
the least or the weakest signal the receiver could accept to be considered satisfactory.
10. FM Improvement (or Practical) Threshold (IT)
IT dBm = -104 + 10logBWMHz + FdB
11. Fade Margin (FM)
FM dB = RSL dBm - IT
dBm
A margin for fading; an allowance (or reservation) in dB, in case the RSL (Received Signal Level) encounters fading.
12. System Gain (SG)
SG = PT(dBm) - IT
dBm
The difference between the nominal output power of a transmitter and the minimum input power required by a receiver.
13. System Reliability
Rayleigh Reliability TableFade
Margin (dB)
Reliability (%)
Outage (%)
8 90 1018 99 128 99.9 0.138 99.99 0.0148 99.999 0.00158 99.9999 0.0001
a. Unavailability (U)
U = MTTR / (MTBF+MTTR)
U = DownTime / TotalTime
b. Reliability (R) or Availability
R= (1 – U) x 100 %
The Outage
For Multi-hop Propagation
Total System Reliability
For multi-hop propagation , the total outage is the summation of each hop and reliability is 100 % - Total Outage. In short, the probability of an equipment or system being operational is: 100% minus the Probability of being non-operational.
The overall system reliability is the product of all individual reliabilities.
Sample problem #1
If the MTBF of a communications circuit is 20,000 hours and its MTTR is 3 hours, what is its availability?
Sample problem #2
A long distance telephone company employs five microwave radio hops over a single route to link two important cities. If each hop has an MTBF of 10,000 hours and an MTTR of 3 hours, what is the MTTR and reliability of the route? Assume that the failure occur at different periods of time.
Are concentric circular zones about the direct path of a microwave signal forming a three-dimensional imaginary solid called an ellipsoid.
Fresnel Zones
1st Fresnel Zone
2nd Fresnel Zone
3rd Fresnel Zone
The radius of the circular zone is in the 1st Fresnel zone, when the reflected path on one-half wavelength longer than the direct path.
1st Fresnel Zone
The radius of the circular zone is in the 2nd Fresnel zone, when the reflected path is two (2) one-half wavelength longer than the direct path, (or one wavelength longer)
2nd Fresnel Zone
The radius of the circular zone is in the 3rd Fresnel zone, when the reflected path is three (3) one-half wavelength longer than the direct path (or 1 ½ wavelength longer).
3rd Fresnel Zone
Radius of the First Fresnel Zone
F1(ft) = 72.1 √((d1(mi) d2(mi)) / (fGHz Dmi))
F1(m) = 17.3 √((d1(km) d2(km)) / (fGHz Dkm))Radius of the nth Zone
Fn = F1 √n
For minimum tower height requirement, design your microwave system to 0.6 of F1, a condition of no gain and no loss.
Sample problem:
A single hop microwave system has the following information:
operating frequency 4 GHzreceive/transmit antenna diameter 3 ft.hop distance 20 milestransmitter output power 1 wattreceiver threshold -78 dBm
Calculate the following:
a. Free space lossb. System gainc. Fade margin and estimated percent reliabilityd. Fresnel zone diameter
Active
Microwave Repeaters
Passive
Provides gain, (55 to 105 dB higher than the received power) and frequency change (252MHz).
intercepts, amplifies and retransmits the signal.
1. Active
Types of Active RepeatersBaseband Repeater
IF Heterodyne Repeater
RF Heterodyne Repeater
Baseband Repeater
It is amplified, demodulated, amplified in the baseband frequency and remodulated.
Offers possibility to drop or insert channel.
Typical output power is 1 watt
IF Heterodyne Repeater
Improved noise performanceTypical output power is 5 watts.
RF Heterodyne Repeater
Amplification is provided directly at microwave frequency.
Bounces the signal from one direction to another.
2. Passive
Types of Passive Repeaters
Billboard
Back to Back Passive
Billboard
Flat metal type used to reflect microwave signals.
Acts as a microwave mirror with no power needed.
Back to Back Passive
Uses two standard antenna dishes directly joined by a short length of waveguide.
Gain of a Passive Repeater
An antenna with good directivity or narrow beamwidth has the reliability of providing directional gain.
English System
Metric System
G dB = 22.2 + 40logf GHz + 20logAft2 + 20cosα
G dB = 42.9 + 40logf GHz + 20logAm2 +
20cosα
Sample problem:
A plane passive reflector 10 x 16 ft. Is erected 21 miles from one active site and only 1 mile from the other. The operating frequency is 2000 MHz. By formula, the free space loss for the longer path is 129.5 dB and for the shorter path, it is 103 dB, calculate the gain of the passive plane reflector and the net path loss if the included angle is 110 degrees.
A method of utilizing 2 or more receivers to reduce fading or increase reliability of the system.
Diversity Reception
Methods of Diversity Reception
Frequency Diversity
Space DiversityPolarization Diversity
Hybrid Diversity
Angle Diversity
Quadrature Diversity
1. Frequency Diversity
Two waves at different frequency travel the same path in a multipath fade.
Signal is transmitted on two (2) different frequencies (properly spaced), over the same path.
CrossBand Diversity – variation of frequency diversity. Frequency separation are entirely of different band allocations.
2. Space Diversity
Signal is transmitted over two different paths (vertically spaced several wavelength apart), on the same frequency.
Advantages of Space Diversity
a. Costlyb. More towers requiredc. Concept does always work as intended
a. Frequency Conservationb. Minimized Multipath Fadingc. Availability of Sufficient Signal Outputd. Compensation for Electrical Differences
Between Direct and Reflected Waves.
Disadvantages of Space Diversity
Vertical Space Between Antennas
Spacing ft = (43.4 λ d) / hT
3. Polarization Diversity
Using dual polarization (vertically and horizontally). Applied to microwave system beyond L-O-S path, (or obstructed path).It requires feedhorn reorientation and is applied to paths beyond LOS as in troposcatter systems.
4. Hybrid Diversity
A special combination of frequency and space diversity.
5. Angle Diversity
Is the transmission of information at two or more slightly different angles resulting to two or more oaths based on illuminating different scatter volumes in troposcatter systems.
6. Quadrature Diversity
The condition where four signals carrying the same information (whose system employs the combination of space or polarization or frequency diversity technique) are available in the receiver, combination of frequency, space and polarization diversity.
Direct Radiating Antenna
Types of Microwave Antennas
Horn Reflected Antenna
Periscope Arrangement
High Performance / Shrouded
Cross Band Parabolic Antenna
a. Direct Radiating AntennaConsist of parabolic antenna with parabolic dish, illuminated by a feed horn at its focus.
Similar to the common parabola, except that they include a cylindrical shield to improve the front-to-back ratio and the wide angle radiation discrimination. Gain efficiency is lower than ordinary parabolic antennas.
b. High Performance / Shrouded
a metal wrapped around the antenna aperture to eliminate side lobes which may cause interference to nearby microwave stations.
Shroud
Radome
a non-metallic (canvass) covering in a parabolic antenna for protection against strong wind velocity. In cold places, ice accumulation is prevented by the use of heated radome.
Permits operation into two widely separate bands. Very complex and critical feed assemblies, have lower gains and poorer VSWR than single band antennas.
c. Cross Band Parabolic
Contains a section of large parabola mounted such as an angle that the energy feedhorn is simultaneously focused and reflected at right angles. It provides a good front-to-back ratio, good VSWR and can be used for multi-band operation on both polarization but offers some moding and distortion problems particularly at higher frequencies.
d. Horn Reflected Antenna
is a combination of a reflector mounted on a tower and the parabolic antenna below. Use the 150 feet and beyond. The spacing between the antenna and the reflector should be in the near field.
f. Periscope Arrangement
1. Rectangular, flat2. Rectangular,
curved3. Elliptical, flat4. Elliptical, curved5. Flyswatter
Shapes of Reflector
General Types of Microwave Tubes
Microwave Components and Devices
Klystron
Magnetron
Travelling Wave Tube (TWT)
Velocity Modulation – The bunching of the electrons within the klystron caused by changing their rate of speed (velocity).
1. Klystron◦An electron tube in
which the electrons are periodically bunched by electric fields. It is used as an oscillator or amplifier in microwave transmitters and receivers.◦ Interaction between an electron beam and an RF voltage.
Two Types of Klystron
Cavity Reflex Klystron
High Power Multicavity Klystron
a. Cavity Reflex Klystron operates as a low power RF oscillator in the microwave region.
b. High Power Multicavity Klystron
two or more cavities, used extensively in fixed radar installations and in UHF television.The size and shape of
multicavity klystron largely determine their operating frequency and power handling capability. smaller klystrons operate at higher frequencies and large klystrons have the higher power handling capability.
2. MagnetronA diode vacuum tube
used as a microwave oscillator in radar and microwave ovens to produce powers up to the megawatt range.
A magnetic field ensures a constant electron beam-RF field interaction.
3. Travelling Wave Tube (TWT)
A microwave power amplifier with very wide bandwidth.
An electric field is used to ensure the interaction between the electron beam and the RF field is continuous.
Other Microwave Tubes
Crossed-Filled Amplifier (CFA)
Backward-Wave Oscillator (BWO)
Twystron
Extended Interaction Amplifier (EIA)
1. Crossed-Filled Amplifier (CFA)
(1) kathode (2) anode with resonant-
cavities (3) Space-Charge Wheel(4) delaying strapping rings
A microwave power amplifier based on the magnetron and looking very much like it. It is a cross between the TWT and the magnetron in its operation.
2. Backward-Wave Oscillator (BWO)
A CW oscillator with an enormous tuning and overall frequency coverage range. It operates on TWT principles of electron beam-RF field interaction, generally using a helix slow – wave structure. It looks like a shorter, thicker, TWT.
3. Twystron
A hybrid combination of klystron driver and TWT output section in tandem with the same envelope.
4. Extended Interaction Amplifier (EIA)
A multicavity klystron with interconnected multigap cavities.
Passive Microwave Circuits
Semiconductor Microwave Devices and Circuits
Stripline
Microstrip
Surface Acoustic Wave (SAW) Devices
1. Stripline
consists of flat metallic ground planes, separated by a thickness of dielectric in the middle of which a thin metallic strip has been buried.
2. Microstrip
has the advantage over stripline in being simpler construction and easier integration with semiconductor devices, lending itself well to printed circuit and thin film techniques.
3. Surface Acoustic Wave (SAW)
use solid piezoelectric materials at frequencies in the VHF and UHF regions.
Microwave Solid State Devices
Point-contact and Schottky or Hot-carrier Diodes
Varactor Diodes or Variable Capacitance Diodes
Step-recovery or Snap-off Diode
Gunn DiodeMetal Semiconductor Field Effect Transistor (MESFET)
IMPATT and TRAPATT
Parametric Amplifier
1. Point-contact and Schottky or Hot-carrier Diodes
Widely used as mixers in microwave equipment as they have low capacitance and inductance.
2. Varactor Diodes or Variable Capacitance Diodes
Widely used as microwave frequency multipliers. Multiplication factors of 2 and 3 are common with power levels up to 20W and efficiencies up to 80%.
3. Step-recovery or Snap-off Diodes
Junction diodes which can store energy in their capacitance and then generate harmonics by releasing a pulse of current.
Are also widely used as frequency multipliers with multiplication factors up to 10 , power ratings up to 50W, and efficiencies approaching 80%.
4. Gunn Diode
A microwave semiconductor device used to generate microwave energy. When combined with a microstrip, stripline, or resonant cavity, simple low power oscillators with frequencies up to 50GHz are easily implemented.
5. MESFET
Replaced parametric amplifiers in the lightweight applications.
(Metal Semiconductor Field Effect Transistor) is used in the microwave band as amplifiers and oscillators.
Recommended