Figure 3-5. Effect of frequency on the critical angle.
(2) The skip zone is a zone of silence between the point where the ground wave becomes too
weak for reception and the point where the sky wave is first returned to Earth. The skip zone's size
depends on the extent of 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.
Occasionally, the first sky wave will return to Earth within range of the ground wave. If the sky and
ground waves are nearly of equal intensity, the sky wave alternately reinforces and cancels the ground
wave, causing severe fading. This is caused by the phase difference between the two waves, which is a
result of the longer path traveled by the sky wave.
f. The relationship between frequency and angle of incidence can be seen in Figure 3-7. You
can see how radio waves reach a receiver via several paths through one layer. The various angles are
represented by dark lines and designated as rays 1 through 6. When the angle of incidence (ray 1) is
relatively low with respect to the horizon, 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 decreases. Note that for rays 4 and 5, the angle is such that the RF
energy penetrates into the central area of maximum ionization of the layer. These rays are refracted
rather slowly and returned to the Earth at great distances. Finally, as the angle approaches vertical
incidence (ray 6), the ray is not returned at all, but passes on through the layer into space.