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When used in the context of antenna construction, radials are physical objects: Wires running away from the base of the antenna, used to augment the conductivity of the ground near the base of the antenna. The radial wires either may run above the surface of the earth, on the surface, or buried a centimeter or so under the earth. 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.
When used in the context of planning for a transmission system, radials 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.
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.
The radials at the antenna base provide a proper ground plane for the types of radio antennas used for long wavelengths. These electrically "short" antennas require grounding or earthing wires to function well. The radials are typically buried in the soil or laid on the soil in a flat, radial pattern.
These wires are called radials, ground radials, grounding radials, ground system radials, or earthing 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.
Similar radiating wires can be placed on the top of antennas (instead of at the base) that serve an almost identical electrical function, but in that case the structure of radial wires at the top end of the antenna is called a capacitance hat or top loading.
When well designed, the 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.
Any metal object within the near field of the radiator must also be tied to this system, or the metal will become energized with radio-frequency voltage, 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.
The use of radial lines on a map for measurement, planning, and regulation of radio transmissions is called the radial method. It has no relation to grounding radials described above.
In the field of transmission 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.
The use of the radial method is more common in North America, where the FCC and CRTC use it in mediumwave transmission planning and regulation. In Europe and Asia, the use of radials has fallen out of favor since the 1970s, and in many nations the radial antenna proof is only acceptable as an ancillary antenna proof. Canada and Mexico, due to lower population densities, never implemented the fully complete radial models that the US FCC did.
The radial method has been falling out of favor for methods based on Cartesian coordinates. Cartesian methods require more CPU time (and memory) to compute, but are understood to more realistically represent antenna systems. The main importance of the radial methods is that a quick antenna system proof can be completed in less than 15 minutes (often in only 5 minutes) of a typical home computer's CPU time, regardless of antenna system complexity.
The ITU over the past 50 years -- in consideration of the various population densities of its members -- officially mandates a minimum of 5 radials for an entire antenna system.
Although many broadcasting regulators around the world had to find some way of regulating longwave and mediumwave antenna patterns and power, only the FCC chose to implement the radial method in its fullest form.
The FCC decision to fully implement radials evolved from 1925 to 1975. Technology had changed, and by the 1980s, computer terrain simulation of station interference and station patterns could be done on mainframes, typically using Cartesian or other non-radial methods.
The FCC rules on radials were relaxed in stages from 1996 to 2013. It is expected that the 2013 ruleset for radials will probably endure without change for a decade.