Sky wave propagation

Sky wave propagation is the mode in which radio waves from a transmitting antenna reach the receiving antenna only after being reflected back down by the ionosphere. This mode covers waves in the frequency range from 3 MHz to 30 MHz, and because such waves can bounce -- undergoing multiple successive reflections between the ionosphere above and the Earth's surface below -- they are able to travel over very large distances, far beyond what ground wave or unaided space wave propagation could reach.

Two technical terms describe the limits of this mode precisely. The critical frequency is the maximum frequency of a radio wave, directed straight (normally) at the ionosphere, that can still be reflected back down to the Earth; waves above this frequency simply pass through the ionosphere into space instead of bouncing back, exactly as noted for the greater-than-30-MHz waves discussed under space wave propagation. The skip distance (or skip zone) is the shortest distance, measured along the Earth's surface from the transmitter, at which a sky wave of a fixed frequency (provided that frequency is above the critical frequency) returns to the Earth -- so no sky wave signal …