(3) The rate at which a wave of a given frequency is refracted by an ionized layer depends on
the angle at which the wave enters the layer. Figure 3-4 shows three radio waves of the same frequency
entering a layer at different angles. The angle at which wave A strikes the layer is too nearly vertical for
the wave to be refracted to Earth. As the wave enters the layer, it is bent slightly but passes through the
layer and is lost. When the wave is reduced to an angle that is less than vertical (wave B), it strikes the
layer and is refracted back to Earth. The angle made by wave B is called the critical angle for that
particular frequency. Any wave that leaves the antenna at an angle greater than the critical angle will
penetrate the ionospheric layer for that frequency and will be lost in space. Wave C strikes the
ionosphere at the smallest angle that can be refracted and still return to Earth. At any smaller angle, the
wave will be refracted but will not return to Earth. As the radio wave's frequency is increased, the
critical angle must be reduced for refraction to occur. This is illustrated in Figure 3-5, page 3-6. The 2-
MHz wave strikes the layer at the critical angle for that frequency and is refracted back to Earth.
Although the 5-MHz wave (broken line) strikes the ionosphere at a lesser angle, it nevertheless
penetrates the layer and is lost. As the angle is lowered from the vertical, however, a critical angle for
the 5-MHz wave is reached, and the wave is then refracted to Earth.
Figure 3-3. Frequency versus refraction
Figure 3-4. Different incident angles of
and distance.
radio waves.
e. The relationship between skip zone, skip distance, and ground wave coverage is shown in
Figure 3-6, page 3-7.
(1) The skip distance is the distance from the transmitter to the point where the sky wave is
first returned to Earth. The skip distance's size depends on the wave's frequency, the angle of incidence,
and the degree of ionization present. Obviously, the skip distance will change through the day as the
level of ionization changes.
3-5
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