Figure 1-7. Direct and reflected routes for radio waves.
If the waves are refracted by the earth, the distance they travel is severely limited due to
large losses of energy in the form of heat dissipated into the earth's crust.
Radio waves may be classified as either ground waves or sky waves (Figure 1-8).
Ground waves are continually in contact with the earth's surface. They do not make use of
reflection from the ionosphere. They have a tendency to be refracted and, in some cases,
reflected into the lower atmosphere. At frequencies above 1500 kilohertz, a ground wave is
affected very little by the time of day or season. The ground wave loses much of its strength
and dissipates energy as it travels over the earth's surface. However, less strength is lost
when it travels over water.
Sky waves are transmitted upward with respect to the earth's surface. Sky waves would not
be useful for communications were it not for the ionosphere. Radio waves approaching the
ionosphere at an angle are refracted back to earth. They may be detected and used for
communications purposes or for DF exploitation.
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