Practical_Antenna_Handbook_0071639586

42 p a r t I I : F u n d a m e n t a l s
Higher level
layer
B
TX
A
RX
C
D
C
Figure 2.22 Multiple signal paths between transmitter and receiver.
present. But each path (including that of Ray A itself) is subject to fading. A moving
vehicle midway between the transmit and receive sites may interrupt path C or C′ during
its travels. In a more complex example, while the moving vehicle is in the ground
reflection path, it may create a new and different geometry for reflection if the incoming
signal hits its roof and reflects off in the direction of the receive antenna! Paths A, B, and
D are all subject to weather-related changes: a gust of wind, a temperature or pressure
gradient moving through the region, raindrops falling, etc.
In general, these mechanisms are frequency-sensitive, so a possible countermeasure
is to use frequency diversity. In many cases, hopping over a 5 percent frequency range will
help eliminate or reduce the fading. If other system constraints and/or local spectrum
usage prevent a 5 percent range, try for at least 2 or 3 percent.
Over oceans or other large bodies of water fair weather surface ducting is sometimes
encountered. These ducts form in the midlatitudes, starting about 2 to 3 km from shore,
up to heights of 10 to 20 mi, with wind velocities in the 10- to 60-km/h range. The resultant
power fading is due to the presence of the duct along with surface reflections (see
Fig. 2.23). Power fading alone can occur when there is a superrefractive duct elevated
above the surface. The duct has a tendency to act as a waveguide and focus the signal
(Fig. 2.24). Although the duct shown is superrefractive, subrefractive ducts are also possible.
Microwave communications paths above about 10 GHz suffer increasingly severe
attenuation caused by water vapor and oxygen in the atmosphere. Figure 2.25 shows
the standard attenuation in decibels per kilometer for microwave frequencies. Note that
in addition to a general upward slope to the graph there are strong peaks at several