Chapter 24: Microwave Radio Communications and System Gain - Review Notes

Reviewer and summary notes of the important concepts and formulas in Chapter 24 of the book "Introduction to Electronics Communications" by Wayne Tomasi.

Chapter 24: Microwave Radio Communications and System Gain

This is the summary notes of the important terms and concepts in Chapter 24 of the book "Electronic Communications System" by Wayne Tomasi. The notes are properly synchronized and concise for much better understanding of the book. Make sure to familiarize this review notes to increase the chance of passing the ECE Board Exam.







Electromagnetic waves with frequencies that range from approximately 500 MHz to 300 GHz or more.



The wavelengths for microwave frequencies, which is than infrared energy.

1 cm and 60 cm slightly longer


The name given to microwave signals, because of their inherently high frequencies, have short wavelengths.

“Microwave” waves


Each frequency is divided in half with the lower half identified as the low band and the upper half as narrow band.

Full-Duplex (Two-way)


Communications system used to carry information for relatively short distances such as between cities with the same state.

Short Haul


Microwave systems that are used to carry information for relatively long distances, such as interstate and backbone route applications.

Long Haul


It propagate signals through Earth’s atmosphere between transmitters and receivers often located on top of tower spaced about 15 miles to 30 miles apart.

Advantages of Microwave Radio:

·         Radio systems do not require a right-of way acquisition between stations.

·         Each station requires the purchase or lease of only a small area of land.

·         Because of their high operating frequencies, microwave radio systems can carry large quantities of information.

·         High frequencies mean short wavelengths, which require relatively small antennas.

·         Radio signals are more easily propagated around physical obstacles such as water and high mountains

·         Fewer repeaters are necessary for amplification.

·         Distances between switching centers are less.

·         Underground facilities are minimized.

·         Minimum delays are introduced.

·         Minimal crosstalk exists between voice channels.

·         Ø Increased reliability and less maintenance are important factors.

Disadvantages of Microwave Radio:

·         It is more difficult to analyze and design circuits at microwave frequencies.

·         Measuring techniques are more difficult to perfect and implement at microwave frequencies.

·         It is difficult to implement conventional circuit components at microwave frequencies.

·         Transient time is more critical at microwave frequencies.

·         It is often necessary to use specialized components for microwave frequencies.

·         Microwave frequencies propagate in a straight line, which limits their use to line-of-sight applications.

Microwave Radios


Propagates signals outside the Earth’s atmosphere and are capable of carrying signals much farther while utilizing fewer transmitters and receivers.

Satellite Systems


It is used in microwave radio systems rather than amplitude modulation because AM signals are more sensitive to amplitude nonlinearities inherent in wideband microwave amplifiers.

Frequency Modulation


Major factor when designing FM Radio systems. It is caused by repeater amplitude nonlinearity in AM, while in FM, it is caused by transmission gain and delay distortion.

Intermodulation Noise


The composite signal that modulates the FM carrier and may comprise one or more of the following:

·         Frequency-division multiplexed voice band channels

·         Time-division-multiplexed voice-band channels

·         Broadcast-quality composite video or picture phone

·         Wideband data



It provides an artificial boost in amplitude to the higher baseband frequencies.

Preemphasis Network


Frequency modulation index used in the FM deviator. Typically, modulation indices are kept between 0.5 and 1.



FM signal that is produces at the output of the deviator with a low-index frequency modulation.

Narrowband FM


A receiver and a transmitter placed back to back or in tandem with the system.

Microwave Repeaters


It receives a signal, amplifies and reshapes it, and then retransmits the signal to the next repeater or terminal station down line from it.

Types of Microwave repeaters:

·         IF

·         Baseband

·         RF

Repeater Station


The received RF carrier is down-converted to an IF frequency, amplified, reshaped, up-converted to an RF frequency, and then retransmitted.

IF Repeater


Generally less than 9 MHz.

Baseband Frequencies


The range id 60 MHz to 80MHz.

IF frequencies


Another name for a Local oscillator, is considerably lower in frequency than either the received or the transmitted radio frequencies.

Shift Oscillator


Transmission used by microwave systems wherein a direct signal path must exist between the transmit and receive antennas.

Line-of Site Transmission


A temporary reduction in signal strength which last in milliseconds for several hours or even days.

Radio Fade


It suggests that there is more than one transmission path or method of transmission available a transmitter and a receiver.



It is simply modulating two different RF carrier frequencies with the same IF intelligence, then transmitting both RF signals to a given destination.

Frequency Diversity


The output of a transmitter is fed to two or more antennas that are physically separated by an appreciable number of wavelengths.

Space Diversity


A single RF carrier is propagated with two different electromagnetic polarizations. It is generally used in conjunction with space diversity.

Polarization Diversity


It is more than one receiver for a single radio-frequency channel. With frequency diversity, it is necessary to also use receiver diversity because each transmitted frequency requires its own receiver

Receiver Diversity


Another form of Hybrid diversity and undoubtly provides the most reliable transmission but most expensive. It combines frequency, space polarization and receiver diversity into one system.

Quad Diversity


A specialized form of diversity that consist of a standard frequency diversity path where the two transmitter/ receiver pairs at one end of the path are separated from each other and connected to different antennas that are vertically separated as in space diversity.

Hybrid Diversity


Alternate facilities temporarily made to avoid a service interruption during periods of deep fades or equipment failures.

Types of protection switching arrangements:

·         hot standby

·         diversity

Protection Switching Arrangement


Each working radio channel has a dedicated backup or spare channel.

Hot Standby Protection


A single backup channel is made available to as many as 11 working channels.

Diversity Protection


In hot standby protection, it splits the signal power and directs it to the working and the spare (standby) microwave channels simultaneously.

Head-End Bridge


It has two working channels, one spare channel, and an auxiliary channel.

Diversity Protection


A low-capacity low-power microwave radio that is designed to be used for a maintenance channel only.

Auxilliary Channel


It is where the number of repeater stations between protection switches depends.

Reliability Objectives of the Systems


Points in the system where baseband signals either originate or terminate.

Four major sections:

·         baseband

·         wireline entrance link (WLEL)

·         FM-IF

·         RF sections

Terminal Stations


Points in a system where baseband signals may be reconfigured or where RF carriers are simply “repeated” or amplified.

Repeater Stations


Stands for WireLine Entrance Link, it serves as the interface between the multiplex terminal equipment and the FM_IF equipment.



A balanced modulator that, when used in conjunction with a microwave generator, power amplifier, and band-pass filter, up-converts the IF carrier to an RF carrier and amplifies the RF to the desired output power.



t must be capable of amplifying very high frequencies and passing very wide bandwidth signals for microwave radios.

devices used in microwave amplifiers:

·         Klystron Tubes

·         Traveling-wave tubes (TWTs)

·         IMPATT (Impact avalanche and transit time)

Power Amplifiers


It provides the RF carrier input to the up-converter. It is called as microwave generator rather than an oscillator because it is difficult to construct a stable circuit that will oscillate in the gigahertz range.

Microwave Generator


It operates in the range 5 MHz to 25 MHz, used to provide a base frequency that is multiplied up to the desired RF carrier frequency.




A unidirectional device often made from ferrite material. It used in conjunction with a channel-combining network to prevent the output of one transmitter from interfering with the output of another transmitter.



Stands for Automatic Gain Control, is a circuit in an IF amplifier.



It occurs only when three stations are placed in a geographical straight line in the system.

Multi-hop Interference


It prevents the power that “leaks” out the back and sides of a transmit antenna from interfering with the signal entering the input of a nearby receive antenna.




The signal entering the input of a nearby receive antenna.

Ring around


It means that these channels are propagated with vertical polarization.

V Channels


The line-of-sight directly between the transmit and receive antenna. Also called as the Direct Wave.

Free-Space Path


It consists of the electric and magnetic fields associated with the currents induced in earth’s surface.

Surface Wave


The portion of the transmit signal that is reflected off Earth’s surface and captured by the receive antenna.

Ground-Reflected Wave


The portion of the transmit signal that is returned back to Earth’s surface by the ionized layers of earth’s atmosphere.

Sky Wave


The loss incurred by an electromagnetic wave as it propagates in a straight line through a vacuum with no absorption or reflection of energy from nearby objects.

Free-Space Path Loss


A phenomenon wherein electromagnetic energy is spread out as it propagates away from the source resulting in lower relative power densities.

Spreading Loss


The reduction in signal strength at the input to a receiver.



The difference between the nominal output power of a transmitter and the minimum input power to a receiver necessary to achieve satisfactory performance.

System Gain


Sometimes called as Link Margin, is essentially a “fudge Factor” included in system gain equations that considers the non ideal and less predictable characteristics of radio wave propagation and terrain sensitivity.

Fade Margin


He described ways of calculating outage time due to fading on a non diversity path as a function of terrain, climate, path length, and fade margin, in April 1969.

W.T. Barnett


From Bell Laboratories, he derived formulas for calculating the effective improvement achievable by vertical space diversity as a function of the spacing distance, path length, and frequency in June 1970.

Arvids Vignant


The ratio of the wideband “carrier” to the wideband noise power.

Carrier-to-Noise Ratio (C/N)


Also called Receiver Sensitivity, is the minimum wide band carrier power at the input to a receiver that will provide a usable baseband output.

Receiver Threshold


The carrier-to-noise ratio before the FM demodulator.

Pre-detection Signal-to-Noise Ratio


The carrier-to-noise ratio after the FM demodulator.

Post detection Signal-to-Noise Ratio


A ratio of input signal-to-noise ratio to output signal to noise ratio.

Noise Factor (F)


The noise factor stated in dB and is a parameter commonly used to indicate the quality of a receiver.

Noise Figure

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