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