SKY WAVE PROPAGATION

Sky wave (or ionospheric) propagation is the mode in which electromagnetic waves radiated from an antenna directed steeply upward, at large angles to the ground, are reflected by the ionosphere back down to Earth, reaching a receiver that may be far beyond the reach of the ground wave. This mode operates over roughly 3 MHz to 30 MHz; waves above about 30 MHz are able to penetrate straight through the ionosphere instead of being reflected, which is why sky wave propagation is specifically exploited for short-wave broadcast services, with the medium- and high-frequency end of this band used for long-distance radio communication. Because the radio waves can undergo multiple successive reflections bouncing back and forth between the Earth and the ionosphere, extremely long-distance communication becomes possible this way -- a single reflection alone lets a wave travel roughly 4000 km. The ionosphere itself is a layer of the upper atmosphere, spread from roughly 50 km to 400 km above the Earth's surface, in which ultraviolet rays, cosmic rays and other high-energy solar and cosmic radiation ionise the air molecules; the resulting charged ions form a reflecting layer that bounces radio waves of the appropriate frequency range back towards the Earth, via what is essentially total internal reflection, so sky waves must be transmitted at a suitable critical angle to guarantee this total reflection rather than escaping out into space. The shortest distance, measured along the Earth's surface, between the transmitter and the nearest point at which the reflected sky wave can be received is called the skip distance. As the angle at which the wave is emitted from the ground is progressively increased, ground-wave reception fades out entirely at some point (marked A); increasing the emission angle further, sky-wave reception only …