TC 9-64 _________________________________________________________________________
Figure 2-20. Ray Paths for a Fixed Frequency with Varying Angles of Incidence
2-76. When the angle is relatively low with respect to the horizon (ray 1),
there is only slight penetration of the layer and the propagation path is long.
When the angle of incidence is increased (rays 2 and 3), the rays penetrate
deeper into the layer but the range of these rays decreases. When a certain
angle is reached (ray 3), the penetration of the layer and rate of refraction are
such that the ray is first returned to Earth at a minimal distance from the
transmitter. Notice, however, that ray 3 still manages to reach the receiving
site on its second refraction (called a hop) from the ionospheric layer.
2-77. As the angle is increased still more (rays 4 and 5), the RF energy
penetrates the central area of maximum ionization of the layer. These rays
are refracted rather slowly and are eventually returned to Earth at great
distances. As the angle approaches vertical incidence (ray 6), the ray is not
returned at all, but passes on through the layer.
ABSORPTION IN THE IONOSPHERE
2-78. Many factors affect a radio wave in its path between the transmitting
and receiving sites. The factor that has the greatest adverse effect on radio
waves is absorption. Absorption results in the loss of energy of a radio wave
and has a pronounced effect on both the strength of received signals and the
ability to communicate over long distances.
2-79. As discussed previously in the section on ground waves, surface waves
suffer most of their absorption losses because of ground-induced voltage. Sky
waves, on the other hand, suffer most of their absorption losses because of
conditions in the ionosphere. Note that some absorption of sky waves may
also occur at lower atmospheric levels because of the presence of water and
water vapor. However, this becomes important only at frequencies above
10,000 megahertz.
2-80. Most ionospheric absorption occurs in the lower regions of the
ionosphere where ionization density is greatest. As a radio wave passes into
the ionosphere, it loses some of its energy to the free electrons and ions. If
these high-energy free electrons and ions do not collide with gas molecules of
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