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 39
tion with ground-wave propagation, but modern practice requires separate treatment.
The older grouping overlooks certain tropospheric propagation phenomena that simply
don’t happen with space or surface waves.
Refraction is the mechanism for most tropospheric propagation phenomena. The
dielectric properties of the air, which are set mostly by the moisture content (Fig. 2.18A
and B), are a primary factor in tropospheric refraction. Recall that refraction occurs in
both light-wave and radio-wave systems when the wave passes between media of differing
density. Under that situation, the wave path will bend in approximate proportion
to the difference in densities.
Two general situations are typically found—especially at UHF and microwave frequencies.
First, because air density normally decreases with altitude, the top of a beam
of radio waves typically travels slightly faster than the lower portion of the beam. As a
result, those signals refract a small amount. Such propagation provides slightly longer
surface distances than are normally expected from calculating the distance to the radio
horizon. This phenomenon is called simple refraction, and was discussed in a preceding
section.
A special case of refraction called superrefraction occurs in areas of the world where
warmed land air flows out over a cooler sea (Fig. 2.19). Typically these areas have deserts
adjacent to large bodies of water: The Gulf of Aden, the southern Mediterranean,
and the Pacific Ocean off the coast of Baja California are examples. Communications up
to 200 mi at VHF/UHF/microwave frequencies are frequently reported in such areas.
The second form of refraction is weather-related. Called ducting, this form of propagation
(Fig. 2.19) is actually a special case of superrefraction. Evaporation of seawater
causes temperature inversion regions—i.e., layered air masses in which the air temperature
is greater than in the layers below it—to form in the atmosphere. (Note: Air
temperature normally decreases with altitude, but at the boundary with an inversion
region it increases.) The inversion layer forms a “duct” that acts like a waveguide. In
Fig. 2.20, the distance D l is the normal radio horizon distance, and D 2 is the distance over
which duct communications can occur.
High
temp
Refracted path
Low
temp
Direct path
Hot desert
Cold sea
Figure 2.19 Superrefraction phenomena.