Microwave Communication

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 microwavefading problems.

Microwave signals bend slightly when passing obliquely through layers of different air density. A microwave signal can be trapped beneath a temperature A microwave communications tower in Munich, Germany. Photograph by Tony Craddock. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. 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 the 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.