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 27
Tropospheric path
XMTR
H t
Tropospheric path
D 2
Earth's surface
D 1 Direct path space wave
Refracted
into ground
D 3 Reflected path space wave
RCVR
Hr
Figure 2.10 Space-wave propagation.
The space wave and surface wave are both ground waves, but they behave differently
enough to warrant separate consideration. The surface wave travels in direct contact
with the earth’s surface and it suffers a severe frequency-dependent attenuation
caused by continual contact with, and absorption by, the earth.
The space wave is also a ground-wave phenomenon, but it is radiated from an antenna
a wavelength or more above the surface. No part of the space wave normally
travels in contact with the surface; VHF, UHF, and microwave signals are usually space
waves. There are, however, two components of the space wave in many cases: direct and
reflected (see Fig. 2.10).
The tropospheric wave is lumped with the direct space wave in some texts, but it
has unique properties in certain practical situations. The troposphere is the region of the
earth’s atmosphere between the surface and the stratosphere, or about 4 to 7 mi above
the surface. Thus, most forms of ground wave propagate in the troposphere. But because
certain propagation phenomena (caused mostly by weather conditions) occur
only at higher altitudes, tropospheric propagation should be differentiated from other
forms of ground wave.
Ground-Wave Propagation
The ground wave, naturally enough, travels along the ground, or at least in close proximity
to it. There are three basic forms of ground wave: space wave, surface wave, and tropospheric
wave. The space wave does not actually touch the ground. As a result, space-wave
attenuation as a function of distance in clear weather is about the same as in free space
(except above about 10 GHz, where H 2 O and O 2 absorption increases dramatically). Of