In RF engineering, radial has three distinct meanings, all referring to lines which radiate from (or intersect at) a radio antenna.
Ground system radial wires When used in the context of antenna construction, radial wires are physical objects: conductors are laid outward from the base of a vertical antenna to form a low-loss ground-return system that minimizes power dissipation in the earth. 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.
Ground system radial wires Stations transmitting at low frequencies like the mediumwave and longwave AM broadcast bands, and some lower shortwave frequencies, have frequencies so low that any feasible antenna is necessarily short compared to the wavelength, the most common being a quarter wave vertical antenna. These wires are called radials, ground radials, grounding radials, ground system radials or earthing radials. The radials at the antenna base provide a proper ground plane for the types of radio antennas used for long wavelengths. These "half dipole" antennas require grounding or earthing wires in order to function well, since the virtual image of the antenna electrically reflected by the mirror-like ground system is an essential part of the operation of the actual antenna standing above the ground system. The radials are typically buried in the soil or laid on the soil in a flat, radial pattern.
Practical issues for ground system radials The ground system radials do not have to be absolutely straight nor absolutely horizontal. Although they provide an electrical "ground", they do not require any actual contact with the surrounding earth, even though advisable.
When the radials are mechanically incorporated into the structure of a small antenna it is called a ground plane antenna. For these antennas the radials slope off at an angle and are also called a skirt. Radials lying upon the ground or within it are not resonant, and there is a great deal of practical latitude in the length and number of radials, although licensing requirements may demand excessive numbers and lengths. Elevated radials are far more efficient at intercepting electrical fields before they reach the earth, so that only three or four may suffice, but since their electrical properties are not dampended by adjacent soil they must be cut to a resonant length.
Electrical hazards When well designed, the far ends of the wires in the ground system carry extremely high voltages. If elevated above the soil, the ends are often connected to ground rods as a safety measure, rather than to improve the function of the antenna. Because of the same hazard, elevated radials are placed at least eight feet above the ground surface, to be out of reach of passers by, or over a fenced area. Any metal object within the near field of the radiator must also be directly tied to the ground system, or the metal will become charged with radio-frequency voltage, and become an electric shock hazard. If large enough to act as a parasitic radiator it may also affect or distort the antenna radiation pattern.
Top loading radial wires Similar radial wires can be placed on the top of antennas (instead of at the base) that also promote more efficient distribution of current in the antenna, but the structure of radial wires added to the top end of the antenna is called a capacitance hat or top loading. Like ground radials, top radials are symmetrically arranged wires placed to radiate away from the apex of the antenna, ideally running horizontally away from the top. Electrically, this is equivalent to attaching a capacitor at the top of the antenna, whose other contact is wired to the ground system, which constitutes the opposite plate of the capacitor. Top loading antennas is a way to effectively increase the height of an antenna (for some purposes, but not all) either to reduce the reactance at the feedpoint, or to indirectly increase radiation resistance by increasing the amount of current in the vertical part of the mast. Although coils can be used to similarly load an antenna, using coils introduces resistance losses from the substantial amount of wire needed; using radials for capacitive loading effectively adds no loss.
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