2-27
Figure 2-21.—Multipath transmission.
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
Fading resulting from multipath propagation is variable with frequency since 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.
Q23. What is the skip zone of a radio wave?
Q24. Where does the greatest amount of ionospheric absorption occur in the ionosphere?
Q25. What is meant by the term "multipath"?
Q26. When a wide band of frequencies is transmitted simultaneously, each frequency will vary in the
amount of fading. What is this variable fading called?
TRANSMISSION LOSSES
All radio waves propagated over ionospheric paths undergo energy losses before arriving at the
receiving site. As we 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 free space
loss. The combined effects of absorption, ground reflection loss, and free space loss account for most of
the energy losses of radio transmissions propagated by the ionosphere.
