lower layer. The lower the frequency of a radio wave, the more rapidly the wave is refracted by a given
degree of ionization. Figure 2-16 shows three separate waves of different frequencies entering an
ionospheric layer at the same angle. Notice that the 5-megahertz wave is refracted quite sharply. The
20-megahertz wave is refracted less sharply and returned to Earth at a greater distance. The
100-megahertz wave is obviously greater than the critical frequency for that ionized layer and, therefore,
is not refracted but is passed into space.
Figure 2-16.Frequency versus refraction and distance.
Angle of Incidence
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 2-17 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 then be lost in space. Wave C strikes the ionosphere at the
smallest angle at which the wave can be refracted and still return to Earth. At any smaller angle, the wave
will be refracted but will not return to Earth.