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.