frequencies that can be used for communications on a given circuit is much smaller than normal, and
communications are possible only at the lower working frequencies.
Q32. What are the two general types of variations in the ionosphere?
Q33. What is the main difference between these two types of variations?
Q34. What are the four main classes of regular variation which affect the extent of ionization in the
Q35. What are the three more common types of irregular variations in the ionosphere?
FREQUENCY SELECTION CONSIDERATIONS
Up to this point, we have covered various factors that control the propagation of radio waves through
the ionosphere, such as the structure of the ionosphere, the incidence angle of radio waves, operating
frequencies, etc. There is a very good reason for studying radio wave propagation. You must have a
thorough knowledge of radio wave propagation to exercise good judgment when you select transmitting
and receiving antennas and operating frequencies. Selection of a suitable operating frequency (within the
bounds of frequency allocations and availability) is of prime importance in maintaining reliable
For successful communications between any two specified locations at any given time of the day,
there is a maximum frequency, a lowest frequency, and an optimum frequency that can be used.
Maximum Usable Frequency
As we discussed earlier, the higher the frequency of a radio wave, the lower the rate of refraction by
an ionized layer. Therefore, for a given angle of incidence and time of day, there is a maximum frequency
that can be used for communications between two given locations. This frequency is known as the
MAXIMUM USABLE FREQUENCY (muf).
Waves at frequencies above the muf are normally refracted so slowly that they return to Earth
beyond the desired location, or pass on through the ionosphere and are lost. You should understand,
however, that use of an established muf certainly does not guarantee successful communications between
a transmitting site and a receiving site. Variations in the ionosphere may occur at any time and
consequently raise or lower the predetermined muf. This is particularly true for radio waves being
refracted by the highly variable F2 layer.
The muf is highest around noon when ultraviolet light waves from the sun are the most intense. It
then drops rather sharply as recombination begins to take place.
Lowest Usable Frequency
As there is a maximum operating frequency that can be used for communications between two
points, there is also a minimum operating frequency. This is known as the LOWEST USABLE
As the frequency of a radio wave is lowered, the rate of refraction increases. So a 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.