overreach

The terrestrial wave propagation of electromagnetic waves depends on the transmitted frequency. In general, all radio waves are subject to diffraction, refraction, reflection and absorption due to topological conditions, whereby the influence on wave propagation can be slight or strong depending on frequency and wavelength.

For example, short and shortest RF waves and microwaves propagate linearly, but longer waves in the meter range are subject to refraction. This is the refraction of electromagnetic waves by layers of air at different temperatures and by ionized layers in the stratosphere. Even longer waves with wavelengths in the kilometer range adapt to the curvature of the earth as ground waves.

The type of wave propagation essentially determines the extent of the radio range. If radio waves are refracted and reflected in the stratosphere, then they can be received on earth at distances many times greater than with linear line-of-sight propagation. This increased radio range is called overreach because it is beyond normal range. It is caused by superrefraction.

The phenomenon of overreach occurs mainly in amateur radio, but also in other terrestrial radio services such as BOS radio, professional radio or marine radio. Amateur radio observes the phenomenon of overreach in the frequency range around 140 MHz with specially installed transmitters and uses the overreach for global communication.