Table 2. Frequency band characteristics.
4.
Propagation in the atmosphere. There are two ways radio energy
travels from the transmitter to the receiver: by means of ground
waves or by sky waves. The ground waves travel along the surface of
the earth. The sky wave travels from the transmitter to one of the
ionospheric layers and is returned to earth.
Long distance radio
communication, depending on the frequency, can be by either ground or
sky wave.
The advantage of sky wave communication is that very
little power is needed to travel long distances, say around 8,000
miles.
In order to communicate by ground waves, a powerful
transmitter is needed in order for the radio waves to travel the same
distances.
A combination of both ground and sky wave communication
usually occurs. The earth's surface affects the radio energy coming
in contact with it.
Terrain features (jungle, desert, and large
bodies of water) either aid or lessen the radio signal. Diffraction
is the bending of the radio wave with the curvature of the earth.
The only variable in a ground wave signal is the terrain over which
it travels.
There are many variables in a sky wave signal: the
frequency, the ionospheric layers, the time of day, the season, and
the sunspot cycle.
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