2-34
Q39. What are some disadvantages of operating transmitters at or near the luf?
Q40. What are some disadvantages of operating a transmitter at or near the muf?
Q41. What is fot?
WEATHER VERSUS PROPAGATION
Weather is an additional factor that affects the propagation of radio waves. In this section, we will
explain how and to what extent the various weather phenomena affect wave propagation.
Wind, air temperature, and 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 since the weather is extremely complex and subject
to frequent change. We will, therefore, limit our discussion on the effects of weather on radio waves to
general terms.
PRECIPITATION ATTENUATION
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, 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.
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 (fig. 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.
