The wave propagation of a space wave is from the antenna into free space. Space waves have shorter wavelengths than ground waves and propagate in a straight line. They occur mainly in the short-wave range and can be reflected back to the earth's surface by atmospheric conducting layers. Because of their propagation they are called Space Wave Propagation.
The shortest distance from the transmitter to the point where a reflected space wave hits the earth's surface again is the space wave skip distance, Sky Wave Skip Distance. The reflection of space waves depends on their wavelength and occurs in the conducting layers of the ionosphere between 50 km and 300 km altitude. The activity of the ionosphere depends on the position of the sun and thus on the time of day.
Atmospheric refraction of the space wave causes the radio horizon to be extended beyond the visible range and beyond the propagation range of the ground wave. Inradio technology, this leads to overreach. Depending on the extension of the radio horizon, one also speaks of super refraction. This occurs during high pressure and inverse weather conditions with reduced humidity. The distance between the point on the earth's surface that can be reached with ground waves and the point where the shortest reflected space wave hits the earth's surface is the dead zone, skip zone. In the skip zone, no radio reception is possible, neither the ground wave nor the space wave.
The space wave is used for shortwave transmissions, but also for over-the-horizon radar( OTH).