TC 9-64 _________________________________________________________________________
Figure 2-21. Multipath Transmission
2-85. Multipath fading may be minimized by practices called space diversity
and frequency diversity. In space diversity, two or more receiving antennas
are spaced some distance apart. Fading does not occur simultaneously at both
antennas; therefore, enough output is almost always available from one of the
antennas to provide a useful signal. In frequency diversity, two transmitters
and two receivers are used, each pair tuned to a different frequency, with the
same information being transmitted simultaneously over both frequencies.
One of the two receivers will almost always provide a useful signal.
Selective Fading
2-86. Fading resulting from multipath propagation varies by frequency
because each frequency arrives at the receiving point via a different radio
path. When a wide band of frequencies is transmitted simultaneously, each
frequency will vary in the amount of fading. This variation is called selective
fading. When selective fading occurs, all frequencies of the transmitted signal
do not retain their original phases and relative amplitudes. This fading
causes severe distortion of the signal and limits the total signal transmitted.
TRANSMISSION LOSSES
2-87. All radio waves propagated over ionospheric paths undergo energy
losses before arriving at the receiving site. As discussed earlier,
absorption in the ionosphere and lower atmospheric levels account for a
large part of these energy losses. There are two other types of losses that
also significantly affect the ionospheric propagation of radio waves. These
losses are known as ground reflection loss and freespace loss. The
combined effects of absorption, ground reflection loss, and freespace loss
account for most of the energy losses of radio transmissions propagated by
the ionosphere.
Ground Reflection Loss
2-88. When propagation is accomplished via multihop refraction, RF energy
is lost each time the radio wave is reflected from the earth's surface. The
amount of energy lost depends on the frequency of the wave, the angle of
incidence, ground irregularities, and the electrical conductivity of the point of
reflection.
2-28
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