Figure 1-16.—Refraction of frequencies below the lowest usable frequency (luf).
As a frequency is lowered, absorption of the radio wave increases. A wave whose frequency is too low is absorbed to such an extent that it is too weak for reception. Atmospheric noise is also greater at lower frequencies. A combination of higher absorption and atmospheric noise could result in an unacceptable signal-to-noise ratio.
For a given angle ionospheric conditions, of incidence and set of the luf depends on the refraction properties of the ionosphere, absorption considerations, and the amount of noise present.
OPTIMUM WORKING FREQUENCY
The most practical operating frequency is one that you can rely onto have the least number of problems. It should be high enough to avoid the problems of multipath fading, absorption, and noise encountered at the lower frequencies; but not so high as to be affected by the adverse effects of rapid changes in the ionosphere.
A frequency that meets the above criteria is known as the OPTIMUM WORKING FREQUENCY It is abbreviated "fot" from the initial letters of the French words for optimum working frequency, "frequence optimum de travail." The fot is roughly about 85% of the muf, but the actual percentage varies and may be considerably more or less than 85 percent.
In this chapter, we discussed the basics of radiowave propagation and how atmospheric conditions determine the operating parameters needed to ensure successful communications. In chapter 2, we will discuss basic antenna operation and design to complete your understanding of radio-wave propagation.
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