TC 9-64 _________________________________________________________________________
2-69. As the frequency of the radio wave is increased, the critical angle must
be reduced for refraction to occur. This point is illustrated in figure 2-18. The
2-megahertz wave strikes the layer at the critical angle for that frequency
and is refracted back to Earth. Although the 5-megahertz 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-megahertz wave is reached, and the wave is then refracted to Earth.
Figure 2-18. Effects of Frequency on the Critical Angle
Skip Distance/Skip Zone
2-70. In figure 2-19, note the relationship between the sky wave skip
distance, the skip zone, and the ground wave coverage. The skip distance is
the distance from the transmitter to the point where the sky wave is first
returned to Earth. The size of the skip distance depends on the frequency of
the wave, the angle of incidence, and the degree of ionization present.
2-71. The skip zone is a zone of silence between the point where the ground
wave becomes too weak for reception and the point at which the sky wave is
first returned to Earth. The size of the skip zone depends on the extent of the
ground wave coverage and the skip distance. When the ground wave coverage
is great enough or the skip distance is short enough that no zone of silence
occurs, there is no skip zone.
2-72. Occasionally, the first sky wave will return to Earth within the range of
the ground wave. If the sky wave and ground wave are nearly of equal
intensity, the sky wave alternately reinforces and cancels the ground wave,
causing severe fading. This fading is caused by the phase difference between
the two waves, a result of the longer path traveled by the sky wave.
2-24
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