public address systems.
Audio frequencies cannot be efficiently radiated from an
antenna. The power requirements are too great and the antenna needed would be much
too long to be practical.
For example, efficient transmission of a 20-Hz audio
signal would require an antenna 4,500 miles long.
b. Radio Frequencies.
The RF range includes all frequencies above the AF
range. Although these frequencies are not audible, they are useful as carriers for
those frequencies that are audible.
Radio waves travel near the earth's surface
and are also radiated skyward at various angles to the earth's surface, as
illustrated in figure 2-1.
Figure 2-1.
(1) The electromagnetic waves travel through space at the speed of light,
approximately 186,000 miles (300,000,000 meters) per second.
(2) The length of a radio wave is the distance traveled by the wave during
the time interval of one complete cycle.
Each complete cycle of two
alternations of the wave is one wavelength.
Wavelength is normally
expressed in meters. The wavelength may be measured from the start of
one wave to the start of the next, as shown in figure 2-2, or from the
crest of one wave to the crest of the next. It is computed by dividing
the velocity by the frequency.
Figure 2-2.
Wavelength of a radio wave.
309 L2
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