TC 9-64 _________________________________________________________________________
The critical angle is the maximum and/or minimum angle at which a radio
wave can be transmitted and still be refracted back to Earth.
Figure 2-Sum 8. Effects of Frequency on Critical Angle
Skip distance is the distance between the transmitter and the point where
the sky wave first returns to Earth.
Skip zone is the 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.
Figure 2-Sum 9. Relationship between Skip Zone, Skip Distance, and Ground Wave