Sky Wave Propagation

Mechanism. In sky-wave propagation, the wave is deliberately directed upward, at an angle, toward the ionosphere rather than along the ground. If its frequency is below that particular ionospheric layer's critical frequency -- the highest frequency the layer is able to reflect back to Earth when the wave strikes it travelling straight up (at vertical incidence) -- the layer bends the wave back down toward the Earth's surface, where it can be picked up by a receiver possibly very far from the transmitter, well beyond the reach of the ground wave. A wave whose frequency is above the critical frequency is not reflected; it simply passes on up through the ionosphere and out into space, lost to any receiver on the ground.

Frequency range and skip distance. This mode is used mainly in the high-frequency (roughly 3 MHz to 30 MHz) band, familiar as short-wave broadcasting, precisely because these frequencies are reflected efficiently by the ionosphere while still being high enough to carry a usable bandwidth. Because the ground wave dies out only a limited distance from the transmitter, while the reflected sky wave lands only much further away (having travelled up to the ionosphere and back down again), there is generally a ring-shaped region on the ground -- the skip zone or skip distance -- lying beyond the ground wave's reach but short of where the sky wave comes back down, in which the transmission cannot be received by either mode at all. …