Figure 2-23.Refraction of frequency below the lowest usable frequency (luf).
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.
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
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
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, absorption, and noise encountered
at the lower frequencies; but not so high as to result in the adverse effects of rapid changes in the
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.
Q36. What do the letters muf, luf, and fot stand for?
Q37. When is muf at its highest and why?
Q38. What happens to the radio wave if the luf is too low?