__________________________________________________________ Radio Wave Propagation
change in density when a radio wave enters the warm air above a duct causes
the wave to be refracted back toward Earth. When the wave strikes the earth
or a warm layer below the duct, it is again reflected or refracted upward and
proceeds on through the duct with a multiple-hop type of action. An example
of the propagation of radio waves by ducting is shown in figure 2-25.
Figure 2-25. Duct Effect Caused by Temperature Inversion
2-132. As the lowest region of the earth's atmosphere, the troposphere
extends from the earth's surface to a height of slightly over 7 miles. Virtually
all weather phenomena occur in this region. Generally, the troposphere is
characterized by a steady decrease in both temperature and pressure as
height is increased. However, the many changes in weather phenomena
cause variations in humidity and an uneven heating of the earth's surface. As
a result, the air in the troposphere is in constant motion. This motion causes
small turbulences, or eddies, to be formed, as shown by the bouncing of
aircraft entering turbulent areas of the atmosphere. These turbulences are
most intense near the earth's surface and gradually diminish with height.
They have a refractive quality that permits the refracting or scattering of
radio waves with short wavelengths. This scattering provides enhanced
communications at higher frequencies.
2-133. Recall that in the relationship between frequency and wavelength,
wavelength decreases as frequency increases and vice versa. Radio waves of
frequencies below 30 megahertz normally have wavelengths longer than the
size of weather turbulences. These radio waves are, therefore, affected very
little by the turbulences. On the other hand, as the frequency increases into
the VHF range and above, the wavelengths decrease in size, to the point that
they become subject to tropospheric scattering. The usable frequency range
for tropospheric scattering is from about 100 megahertz to 10 gigahertz.
Tropospheric Scattering
2-134. When a radio wave passing through the troposphere meets
turbulence, it makes an abrupt change in velocity. This causes a small
amount of the energy to be scattered in a forward direction and returned to
Earth at distances beyond the horizon. This phenomenon is repeated as the
radio wave meets other turbulences in its path. The total received signal is an
accumulation of the energy received from each of the turbulences.
2-37
Previous Page
