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
1 cm and 60 cm slightly longer
The name given to microwave signals, because of their inherently
high frequencies, have short wavelengths.
Each frequency is divided in half with the lower half
identified as the low band and the upper half as narrow band.
Communications system used to carry information for relatively
short distances such as between cities with the same state.
Microwave systems that are used to carry information for
relatively long distances, such as interstate and backbone route
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:
systems do not require a right-of way acquisition between stations.
station requires the purchase or lease of only a small area of land.
of their high operating frequencies, microwave radio systems can carry large
quantities of information.
frequencies mean short wavelengths, which require relatively small antennas.
signals are more easily propagated around physical obstacles such as water
and high mountains
repeaters are necessary for amplification.
between switching centers are less.
facilities are minimized.
delays are introduced.
crosstalk exists between voice channels.
Increased reliability and less maintenance are important factors.
Disadvantages of Microwave Radio:
is more difficult to analyze and design circuits at microwave frequencies.
techniques are more difficult to perfect and implement at microwave
is difficult to implement conventional circuit components at microwave
time is more critical at microwave frequencies.
is often necessary to use specialized components for microwave frequencies.
frequencies propagate in a straight line, which limits their use to line-of-sight
Propagates signals outside the Earth’s atmosphere and are
capable of carrying signals much farther while utilizing fewer transmitters
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.
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.
The composite signal that modulates the FM carrier and may
comprise one or more of the following:
multiplexed voice band channels
composite video or picture phone
It provides an artificial boost in amplitude to the higher
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.
A receiver and a transmitter placed back to back or in tandem
with the system.
It receives a signal, amplifies and reshapes it, and then
retransmits the signal to the next repeater or terminal station down line
Types of Microwave repeaters:
The received RF carrier is down-converted to an IF frequency,
amplified, reshaped, up-converted to an RF frequency, and then retransmitted.
Generally less than 9 MHz.
The range id 60 MHz to 80MHz.
Another name for a Local oscillator, is considerably lower in
frequency than either the received or the transmitted radio frequencies.
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.
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
The output of a transmitter is fed to two or more antennas
that are physically separated by an appreciable number of wavelengths.
A single RF carrier is propagated with two different
electromagnetic polarizations. It is generally used in conjunction with space
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
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.
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.
Alternate facilities temporarily made to avoid a service
interruption during periods of deep fades or equipment failures.
Types of protection switching arrangements:
Protection Switching Arrangement
Each working radio channel has a dedicated backup or spare
Hot Standby Protection
A single backup channel is made available to as many as 11
In hot standby protection, it splits the signal power and
directs it to the working and the spare (standby) microwave channels
It has two working channels, one spare channel, and an
A low-capacity low-power microwave radio that is designed to
be used for a maintenance channel only.
It is where the number of repeater stations between protection
Reliability Objectives of the Systems
Points in the system where baseband signals either originate
Four major sections:
entrance link (WLEL)
Points in a system where baseband signals may be reconfigured
or where RF carriers are simply “repeated” or amplified.
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:
(Impact avalanche and transit time)
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
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
It occurs only when three stations are placed in a
geographical straight line in the system.
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.
It means that these channels are propagated with vertical
The line-of-sight directly between the transmit and receive
antenna. Also called as the Direct Wave.
It consists of the electric and magnetic fields associated
with the currents induced in earth’s surface.
The portion of the transmit signal that is reflected off
Earth’s surface and captured by the receive antenna.
The portion of the transmit signal that is returned back to
Earth’s surface by the ionized layers of earth’s atmosphere.
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
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
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
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.
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.
The ratio of the wideband “carrier” to the wideband noise
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
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 Factor (F)
The noise factor stated in dB and is a parameter commonly used
to indicate the quality of a receiver.