In radio telecommunications, effective radiated power or ERP is determined by subtracting system losses from system gains. ERP is typically applied to antenna systems. For example, if an antenna system has +9 dB gain and −6 dB loss, its ERP is +3 dB over the transmitter power output (TPO).
For example, an FM radio station which advertises that it has 100,000 watts of power actually has 100,000 watts ERP, and not a 100,000-watt transmitter. The TPO of such a station is most likely around 10,000-20,000 watts, with a gain of +7 to +10 dB (5× to 10×). In addition, if the antenna is directional, the ERP figure is usually the maximum in any one direction, rather than the average.
ERP is always relative to a reference antenna. A dipole antenna is most often used, but a notional isotropic antenna (one which radiates equally in all directions) may also be used, and frequently is in respect of satellite transponders. (When measured in the latter way, the initialism EIRP is used.) Informally, in the case of mediumwave (AM) stations in the United States, ERP is sometimes used; in this case it is computed relative to an omnidirectional radiator with the same nominal power and an efficiency equal either to the RMS efficiency of the directional antenna under consideration, or to the minimum efficiency permitted for the class of station.
In the United States, the maximum ERP for FM broadcasting is usually 100,000 watts (FM Zone II) or 50,000 watts (FM Zone I/I-A), though exact restrictions vary depending on the class of license. Some stations have been grandfathered in or, very infrequently, been given special dispensation, and can exceed normal restrictions (a significant example is WBCT of Grand Rapids, Michigan, which is licensed at 320,000 watts ERP). Height above average terrain (HAAT) is also a factor in determining the broadcast range of a station. Licenses granted by the Federal Communications Commission are based on ERP, antenna height, and range, with range ultimately being the most significant factor. For example, some stations exceed normal height restrictions and therefore must downgrade ERP in order to fit within the appropriate range.
Effective radiated power:
1. The power supplied to an antenna multiplied by the antenna gain in a given direction.
- Note 1: If the direction is not specified, the direction of maximum gain is assumed.
- Note 2: The type of reference antenna must be specified.
2. The product of the power supplied to the antenna and its gain relative to a half-wave dipole in a given direction (used by the NTIA and FCC).
- Note: If the direction is not specified, the direction of maximum gain is assumed.
Effective isotropically-radiated power (EIRP):
The arithmetic product of (a) the power supplied to an antenna and (b) its gain relative to an isotropic source.