Microwave Communication
Microwave Communication
Microwaves are radio signals with a very short wavelength. Microwave signals can be focused by antennas just as a searchlight concentrates light into a narrow beam. Signals are transmitted directly from a source to a receiver
site. Reliable microwave signal range does not extend very far beyond the visible horizon.
If microwave signals were visible to the eye, cities would be seen to be crisscrossed by microwave transmissions carrying important signals. Any type of information that can move over telephone wires or coaxial cables can be transmitted over a microwave circuit as efficiently as through the wires and cables they supplement.
Microwaves and power
The ability to focus microwave signals into narrow beams results in very high antenna gain. Antenna gain increases the effective-radiated power of a microwave signal much as the reflector in a flashlight produces a tight beam of light powerful enough to illuminate distant objects. The most common microwave-antenna focuses the signal by reflecting it from a parabolically-curved reflecting surface sometimes called a dish.
High antenna gain means that microwave transmitters need not be extremely powerful to produce a strong signal. A transmitter rated at 10 watts or less, using an antenna that concentrates the signal toward its target, can produce a received signal as strong as if thousands of watts were scattered in all directions.
Microwave transmitters
The lower-powered microwave signals used by communication transmitters are usually produced by solid-state devices. The Gunn diode is an example. When supplied with voltage from a well-regulated power supply these devices reliably produce a few watts of microwave signal.
Spatial diversity
A sharply defined beam of microwave transmissions means that separate signals can use the same range of frequencies without mutual interference if the paths are carefully planned. Lower radio-frequency assignments can accommodate fewer users because longer-wavelength signals cannot be confined to a narrow beam. A relative immunity to interference allows each microwave signal to use a very wide bandwidth. Wider bandwidths are required when more information per second is transmitted.
It is common practice to locate microwave receivers and transmitters atop high buildings when hilltops or mountain peaks are not available. The higher the antennas are raised, the further will be the distance to the radio horizon. It takes many ground-based relay “hops” to carry a microwave signal across a continent. Since the 1960s the United States has been spanned by a network of microwave relay stations.
Satellites and microwaves
Earth satellites relaying microwave signals from the ground have increased the distance that can be covered in one hop. Microwave repeaters in a satellite in a stationary orbit 22,300 mi (35,881 km) above Earth is high enough to reach one third of Earth’s surface. Microwave signals can be relayed by just one satellite repeater when that satellite is simultaneously above the horizon for both Earth-bound transmitter and the receiver.
Microwave propagation
Microwave signals usually travel from transmitter to receiver along nearly straight-line paths. There are occasional exceptions. The same atmospheric conditions that cause optical mirages can cause microwave-fading problems. Microwave signals bend slightly when passing obliquely through layers of different air density. A microwave signal can be trapped beneath a temperature inversion, causing a strong signal to fade when the signal cannot reach a receiver atop a mountain peak. Atmospheric ducting can cause a microwave signal to follow the curvature of Earth so that it reaches far beyond the horizon. Radar signals at microwave frequencies may reveal the presence of surface ships at distances of hundreds of miles but be unable to display radar returns from aircraft in flight. This happens unpredictably but the problems usually persist for only short periods.
Microwave signals are reflected by flat surfaces. Plane reflectors may be used to bounce a microwave signal around a hill or a building that would otherwise block its path. Flat reflectors are often placed at the top of tall microwave-relay towers. Parabolic dish antennas at ground level face skyward, directed toward the reflectors that bounce their signals to the horizon.
A passive microwave reflector needs little maintenance and requires no power. Their principle drawback is that the strength of the microwave signal drops off as the inverse fourth power of the total distance when it has been reflected from a passive repeater, greatly increasing the path loss. Doubling the total distance reduces received-signal power by one sixteenth.
All wave-based phenomena interact strongly with objects having a size comparable to a wavelength. Raindrops and hailstones are similar in size to the wavelength of higher-frequency microwaves. A rainstorm can block microwave communication producing a condition called rain fade. Weather radar deliberately uses shorter-wavelength microwaves to increase interaction with rain.
Microwave path loss
Microwave communications systems must be carefully engineered if they are to provide reliable communications. Engineers can predict the signal loss in decibels (dB) for a given signal path. They are then able to specify the transmitter power, total antenna gain, and receiver sensitivity required for the circuit. Additional signal or gain can be specified to protect against most fades.
As an example, suppose that a proposed path for a microwave relay system between a radio-station studio and a remote transmitter site is predicted to have a
KEY TERMS
Bandwidth— Range of frequencies available for a dedicated purpose.
Coaxial cable— A concentric cable, in which the inner conductor is shielded from the outer conductor; used to carry complex signals.
Decibel— A unit of measurement of the intensity of sound, abbreviated dB.
Ducting— Trapping a radio signal so it follows Earth’s curvature.
Inversion— An atmospheric condition in which air temperature increases with increasing altitude, instead of the usual decrease.
Parabolic— Shape based on the parabola, a conic section from mathematics.
Passive reflector— Surface used to reflect a signal.
Propagation— Basis for the transmission of a signal.
Radar— A method of detecting distant objects based on the reflection of radio waves from their surfaces.
Spatial diversity— Reduction of interference by maintaining physical separation.
circuit loss of 110 dB. If the system design provides a safety cushion of 30 dB against the possibility of rain fade, the total gains and losses in the system must equal at least +140 dB. If the calculations fall short of this target by 10 dB, for example, the engineer can increase the power of the transmitter by a factor of 10, or increase the gain of one or both of the other antennas by 10 dB, or increase the receiver sensitivity, by 10 dB. Any combination of improvements that will add to 10 dB will provide the desired performance.
Microwave communication is nearly 100% reliable, in part because the circuits have been engineered to minimize fading and in part because computer-controlled networks often reroute signals through a different path before a fade becomes noticeable.
Resources
BOOKS
Golio, Mike. RF and Microwave Applications and Systems. New York: CRC, 2007.
Pozar, David M. Microwave Engineering. New York: Wiley, 2004.
Donald Beaty