Radial (radio)

In RF engineering, radial has two different meanings, both referring to lines which radiate from (or intersect at) a radio antenna.

Planning

In the field of planning, radials are evenly-spaced points (vectors) along evenly-spaced lines (bearings) from a common point on a map, which are used to determine the average elevation above mean sea level (AMSL) within a radio station's broadcast range (including broadcast stations and cellphone base stations, among others). This in turn determines the station's height above average terrain (HAAT), which greatly affects its coverage area (more so than effective radiated power), and therefore the potential for RF interference with other adjacent stations or cells. This information must be submitted with an application for a construction permit. The points used for calculation may differ if a directional antenna is used.

Grounding

Stations at low frequencies like shortwave, and especially the mediumwave/longwave AM broadcasting bands, require grounding or earthing wires to be buried in the soil in a radial pattern, in order to have a proper ground plane for the large mast radiators used as radio antennas at their long wavelengths. These wires are also called radials, ground radials, grounding radials, or earthing radials. All metal objects within the near field of the radiator must also be tied to this system, or they will be energized with radio-frequency energy, and become an electric shock hazard, as well as potentially affecting or distorting the antenna pattern as a parasitic radiator. In one unusual case, the strip mall built around the WSB AM tower near Atlanta has every metal object (such as plumbing and ductwork) grounded for this reason.