(3) The F region ranges from about 90 to 240 miles high. During daylight hours, the F
region separates into two layers-the F1 and F2 layers. At night these two layers combine.
Recombination occurs slowly after sunset, so a fairly constant ionized layer is present at all times. The
F layers are very useful for HF long-distance radio communications.
d. A radio wave transmitted into an
ionized layer is refracted (bent) as it abruptly
changes velocity while entering a new medium.
(1) The relationship between radio
waves and ionization density is shown in Figure
3-2. Each layer has a central region of relatively
dense ionization which tapers off in intensity
both above and below the maximum region. As
a radio wave strikes a region of increased
ionization, its velocity increases, causing it to
bend back toward the Earth. If a radio wave
strikes a thin, very highly ionized layer, the
wave may be bent back and appear to have been
Figure 3-2. Effects of ionospheric density on
reflected, rather than refracted back to Earth.
radio waves.
Ionospheric reflection is more likely to occur at
long wavelengths (low frequencies). This is what occurs when you bounce an AM signal off the
ionosphere and it is picked up many hundreds of miles away.
(2) For any given time, each ionospheric layer has a maximum frequency at which radio
waves can be transmitted vertically and refracted back to Earth. This frequency is called the critical
frequency. Radio waves transmitted at frequencies higher than the critical frequency of a given layer
will pass through the layer and be lost in space. If this wave enters into an upper layer with a higher
critical frequency, the wave will be refracted back to Earth. Radio waves of frequencies lower than the
critical frequency will also be refracted back to Earth, unless they are absorbed or have been refracted
from a lower layer. The lower the frequency of a radio wave, the more rapidly the wave is refracted by
a given degree of ionization. Figure 3-3 shows three separate waves of different frequencies entering a
ionospheric layer at the same angle. Notice that the 5-MHz wave is refracted quite sharply. The 20-
MHz wave is refracted less sharply and returned to Earth at a greater distance. The 100-MHz wave is
obviously greater than the critical frequency for that ionized layer. Therefore, it is not reacted but is lost
in space.
SS0130
3-4
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