Radio horizon

From Glossary of Meteorology

radio horizon

The locus of points at which direct rays from a radio transmitter become tangential to the earth's surface.

The radio horizon extends beyond the geometric and visible horizons in conditions of normal atmospheric refraction. It may be decreased or increased in particular cases as standard propagation is replaced by substandard propagation or superstandard propagation, respectively. Beyond the radio horizon, surface targets cannot be detected under VHF atmospheric conditions although significant radio power is sometimes detected in the diffraction zone below the horizon. This power is a result of scattering by turbulence-produced atmospheric inhomogeneities. Assuming a smooth surface, the distance of the radio horizon is given approximately by the equation
, where R is the distance in kilometers and h is the height in meters of the antenna above the surface.
See effective earth radius, scatter propagation.