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In RF engineering, radial has three distinct meanings, both referring to lines which radiate from (or intersect at) a radio antenna, but neither meaning is related to the other.
- Ground system radial wires
When used in the context of antenna construction, radial wires are physical objects: Wires running away from the base of the antenna, used to augment or replace the conductivity of the ground near the base of the antenna. The radial wires either may run above the surface of the earth (elevated radials), on the surface (on ground radials), or buried a centimeter or so under the earth (buried radials). The ends of the wires nearest the antenna base are connected to the antenna system electrical ground, and the far ends are either unconnected, or connected to metal stakes driven into the earth.
- Top loading radial wires
Symmetrically arranged radial wires may also be attached to the top of an antenna, running horizontally away from its apex. For practical length radials, their effect is to improve feedpoint impedance of a short antenna almost the same as extending the height of the antenna by an amount equal to the combined length of all the radials, up to a point of diminishing returns around about a dozen radials. The radials do not themselves radiate, but may indirectly cause a small improvement in antenna radiation of short antennas by raising their point of maximum current upward along the main part of the mast.
- Map radial lines
When used in the context of planning for a transmission system, radial lines are a concept used when describing a radio station's broadcast range: The radials in this case are several lines drawn on a map, radiating from the transmitter, with evenly spaced horizontal bearings. The radial extends as far as the transmitted signal can reach either by calculation or by measurement.