__________________________________________________________ Radio Wave Propagation
storms have occurred when there were no visible spots on the sun and no
preceding SID. As you can see, some correlation between ionospheric storms,
SID, and sunspot activity is possible, but there are no hard and fast rules.
Ionospheric storms can occur suddenly without warning.
2-113. The most prominent effects of ionospheric storms are a turbulent
ionosphere and very erratic sky wave propagation. Critical frequencies are
lower than normal, particularly for the F2 layer. Ionospheric storms affect
the higher F2 layer first, reducing its ion density. Lower layers are not
appreciably affected by the storms unless the disturbance is great. The
practical effect of ionospheric storms is that the range of 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.
2-114. 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, and operating frequencies.
There is a very good reason for studying radio wave propagation. One must
have a thorough knowledge of radio wave propagation to exercise good
judgment when selecting 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
Note. For successful communications between any two specified locations
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
2-115. As 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).
2-116. 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
2-117. As there is a maximum operating frequency that can be used for
frequency. This is known as the lowest usable frequency (LUF). As the
frequency of a radio wave is lowered, the rate of refraction increases. So a