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A large number of things—some desirable, some not—can happen to a radio wave on its way from one place to another. In the simplest case two antennas (a transmitter and a receiver) are located so that a direct path connects them. Waves at most radio frequencies will experience no difficulties in transit beyond a normal attenuation as the wave spreads itself through space and with the usual losses from resistance of the medium—air, in this case.

But broadly speaking, the earth is a radio environment strewn with obstacles: an atmosphere that thins with altitude, becoming finally a charged shell (the ionosphere), and of changeable composition (clouds, rain, hot and cold air); a surface studded with irregularities, from mountains to city buildings, not to mention the curvature of the planet itself; and a mix of land and sea areas, with different conductive and reflective properties. Fortunately, the effect on propagating waves is very much dependent on frequency.

Longer waves get around obstacles well enough; like ocean waves meeting a narrow pier, they just flow around. They diffract over the horizon, too. That is, they are spreading themselves widely enough to stay in contact with the earth as they move outward. The earth absorbs higher frequencies rapidly, however, so waves that are to be received deep in the "shadow zone" would need to be a half-mile or so long, about 300 kHz, which is too low, say, for good voice transmission. To some degree earth conducts these waves, as well, changing their properties over distance.

Waves below 300 kHz can't penetrate the ionosphere at all and consequently are trapped in a zone between the surface and the charged layer above. They are conducted by this natural "waveguide" around the earth, losing energy with distance until, remarkably, a small surge can be measured where the wave meets and reinforces itself on the other side of the globe.

Most of the spectrum chosen for communications of any sort are frequencies that either spread just enough over the horizon (as in AM broadcasting), or must have line-of-sight connection (as in FM), or reflect back from the ionosphere (the "sky waves" of shortwave radio). The shorter the wavelength, however, the more likely it will experience scattering or absorption by obstacles, moisture, or the earth. Satellites beaming broadcasts back to the surface use efficient, line-of-sight, ultra-short wavelengths—but rain or heavy fog can absorb them.


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