Effective radiated power
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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, a 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 (transmitter power output) of such a station is most likely around 10,000-20,000 watts, with a gain of +7 to +10 dB (5× to 10×). Since to have any gain an antenna must be to some extent directional the ERP will also vary by direction and like the antenna gain the maximum is usually quoted. ERP for FM radio in the United States is always relative to a reference half-wave dipole antenna. (Double the TPO if the station broadcasts with circular polarization (CP), that is, with equal horizontal and vertical signal components; it takes twice as much transmitter power for CP.) This paragraph applies to analog TV also; it only generally applies to digital TV and digital FM.
For most microwave systems, a completely non-directional isotropic antenna (one which radiates equally and perfectly well in every direction - a physical impossibility) is usually used as a reference antenna. This includes satellite transponders, radar, and other systems which use microwave dishes and reflectors rather than dipole-style antennas. (When referencing the theoretical isotropic antenna, the abbreviation EIRP is used.) Although it is physically impossible to make an isotropic antenna, it makes the math a bit simpler.
In the case of mediumwave (AM) stations in the United States, actual radiated power is used for an omnidirectional station; for a directional station, power 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 Europe, sometimes the term effective monopole radiated power or EMRP is used, especially in relation to mediumwave broadcasting antennas. This is the same as ERP, except that a quarter-wavelength monopole is used as the reference antenna instead of a half-wave dipole.
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. 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.
Other countries do not share the same power limits as the United States, but the physical limits of radio broadcasting still apply. For example, CITI's ERP of 360,000 watts makes it the most powerful FM station in the world, but its range is not appreciably further than a 100,000 watt station.
See also
From FS-1037C:
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.de:Effective Radiated Power pl:ERP (moc)