Practical_Antenna_Handbook_0071639586

C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 41
Low
temp
Higher
temp
Direct path
Refracted path
Cold land mass
Warmer sea
Figure 2.21 Subrefraction phenomena.
All tropospheric propagation dependent upon air-mass temperatures and humidity
shows diurnal (i.e., over the course of the day) variation caused by the local rising and
setting of the sun. Distant signals may vary 20 dB in strength over a 24-hour period.
These tropospheric phenomena explain how TV, FM broadcast, and other VHF signals
can propagate great distances—especially along seacoast paths—some of the time,
while being weak or undetectable at others.
Fading Mechanisms
Fading is the name given to the perceptible effects of variations over time in received
amplitude and/or phase. These variations are assumed to originate in changing propagation
effects between the transmitter and the receiver, rather than as a result of changes
in transmitter output power. Some of the most common sources include weatherrelated
path loss, multipath anomalies, vehicles and other objects in motion, and variations
in the height and density of the ionospheric layer of the atmosphere.
You might not expect line-of-sight radio relay links to experience fading, but they
do. Fading does, in fact, occur, and it can reach levels of 30 dB in some cases (20 dB is
relatively common). In addition, fading phenomena in the VHF-and-up range can last
several hours, with some periods of several days (although very rare) reported.
Generally, fading is the result of varying path loss and/or phase shift on at least one
path between the transmitting (TX) and receiving (RX) antennas. In Fig. 2.22, Ray A
represents the direct path that, ideally, is the only path between source and destination.
But other paths are also possible. Ray B is an example of energy from the same transmitting
antenna traveling through an elevated layer or other atmospheric anomaly capable
of providing refraction or subrefraction of the transmitted wave. Ray C adds signal via
a ground reflection path, and Ray D is an example of yet another wave from the transmitting
antenna arriving at the receiver via subrefraction fading.
All three of Rays B, C, and D will have amplitudes and phases that differ from those
of Ray A. One or more may, for instance, be totally out of phase with the received voltage
from Ray A. Unless one or more of them varies over time, fading is not likely to be