__________________________________________________________ Radio Wave Propagation
EFFECTS OF WEATHER ON PROPAGATION
2-123. Weather is an additional factor that affects the propagation of radio
waves. In this section, we explain how and to what extent the various
weather phenomena affect wave propagation.
2-124. Wind, air temperature, and the water content of the atmosphere can
combine in many ways. Certain combinations can cause radio signals to be
heard hundreds of miles beyond the ordinary range of radio communications.
Conversely, a different combination of factors can cause such attenuation of
the signal that it may not be heard even over a normally satisfactory path.
Unfortunately, there are no hard and fast rules on the effects of weather on
radio transmissions because the weather is extremely complex and subject to
frequent change. Therefore, we limit our discussion on the effects of weather
on radio waves to general terms.
Precipitation Attenuation
2-125. Calculating the effect of weather on radio wave propagation would be
comparatively simple if there were no water or water vapor in the
atmosphere. However, some form of water (vapor, liquid, or solid) is always
present and must be considered in all calculations. Before we begin
discussing the specific effects that individual forms of precipitation (rain,
snow, and fog) have on radio waves, you should understand that attenuation
because of precipitation is generally proportionate to the frequency and
wavelength of the radio wave. For example, rain has a pronounced effect on
waves at microwave frequencies. However, rain hardly affects waves with
long wavelengths (HF range and below). You can assume, then, that as the
wavelength becomes shorter with increases in frequency, precipitation has an
increasingly important attenuation effect on radio waves. Conversely, you
can assume that as the wavelength becomes longer with decreases in
frequency, precipitation has little attenuation effect.
2-126. Rain. Attenuation because of raindrops is greater than attenuation
because of other forms of precipitation. Attenuation may be caused by
absorption, in which the raindrop, acting as a poor dielectric, absorbs power
from the radio wave and dissipates the power by heat loss or by scattering
(figure 2-24). Raindrops cause greater attenuation by scattering than by
absorption at frequencies above 100 megahertz. At frequencies above
6 gigahertz, attenuation by raindrop scatter is even greater.
2-35
Previous Page
