groundwave

Groundwave, also known as surface wave, is a radio wave propagation mode that travels along the surface of the Earth, following the planet’s curvature. It is guided by the boundary between air and ground and penetrates only shallowly into the surface, with energy concentrated near the surface. Groundwave propagation is strongest at low frequencies, particularly in the longwave and mediumwave bands, and extends the range of transmission beyond line of sight.

The attenuation of a groundwave depends on frequency and the electrical properties of the ground, especially conductivity and relative permittivity. Higher conductivity and moisture reduce losses, allowing longer reach, while dry, rocky soils increase attenuation. The wave speed is close to the speed of light but is slightly reduced by the ground’s loading. At very low frequencies, groundwave can travel long distances, whereas at higher frequencies it becomes rapidly attenuated and is eventually dominated by skywave propagation, which relies on ionospheric reflections.

Groundwave is distinct from skywave, which travels by multiple reflections from the ionosphere and can cover intercontinental distances. It is also different from line-of-sight propagation, which is limited by the horizon.

Applications include AM broadcasting and some LF/MF communications. Groundwave is most effective at lower frequencies and over highly conductive surfaces such as oceans; its usefulness diminishes at higher frequencies where ionospheric propagation becomes the primary long-range mechanism. It is influenced by terrain, sea state, and atmospheric conditions.