TC 9-64 _________________________________________________________________________
wave whose frequency is below the established LUF is refracted back to
Earth at a shorter distance than desired, as shown in figure 2-23.
Figure 2-23. Refraction of Frequency below the Lowest Usable Frequency (LUF)
2-118. The transmission path that results from the rate of refraction is not
the only factor that determines the 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. Likewise,
atmospheric noise is greater at lower frequencies; thus, a low-frequency radio
wave may have an unacceptable signal-to-noise ratio.
2-119. For a given angle of incidence and set of ionospheric conditions, the
LUF for successful communications between two locations depends on the
refraction properties of the ionosphere, absorption considerations, and the
amount of atmospheric noise present.
Optimum Working Frequency
2-120. Neither the MUF nor the LUF is a practical operating frequency.
While radio waves at the LUF can be refracted back to Earth at the desired
location, the signal-to-noise ratio is still much lower than at the higher
frequencies, and the probability of multipath propagation is much greater.
Operating at or near the MUF can result in frequent signal fading and
dropouts when ionospheric variations alter the length of the transmission path.
2-121. The most practical operating frequency is one that you can rely on
with the least amount of problems. It should be high enough to avoid the
problems of multipath propagation, absorption, and noise encountered at the
lower frequencies; but not so high as to result in the adverse effects of rapid
2-122. A frequency that meets the above criteria has been established and 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 percent of the MUF but
the actual percentage varies and may be either considerably more or less
than 85 percent.