In radio telecommunications, effective radiated power (ERP) is determined by subtracting system losses and adding system gains to the actual electrical power output of a transmitter. ERP is not equivalent to the power that is radiated, but is a quantity that takes into consideration transmitter power and antenna directivity. 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 probably not an actual 100,000-watt transmitter. The TPO (transmitter power output) of such a station is perhaps around 10,000 to 20,000 watts, with a gain factor of, in this case, 5 to 10 (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.
US regulatory usage
ERP for FM radio in the United States is always relative to a theoretical 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.
In the United States, the maximum ERP for FM broadcasting is usually 100,000 watts (FM Zone II) or 50,000 watts (the more densely populated FM Zones 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.
Microwave band issues
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, the assumption makes calculations simpler.
Lower frequency issues
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.
HAAT vs 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 radiation pattern, with range ultimately being the limiting factor. For example, some stations exceed normal height restrictions and therefore must downgrade ERP in order to fit within the appropriate range and thereby protect other stations using the same or neighbouring frequencies.
It should be noted here that, unlike with the so-called "AM" broadcast band (mediumwave), which uses far lower frequencies, power or ERP on an FM broadcast station cannot by itself indicate the station's range; the height of its antenna is crucial. A station may put out a megawatt but if it is situated in a low valley surrounded by mountains, the signal will be contained therein. This action is called Line-of-sight propagation in VHF (FM) Radio Communications.
On the other hand, given a sufficiently high perspective, a transmitter needs little power to cover a significant area. In the extreme case, a communications satellite orbiting with unobstructed view of its terrestrial target may need only 5 to 10 watts, even though it is thousands of miles away. This is why broadcast FM (and TV) transmitters are placed on high towers or mountains for best coverage.
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.
- Note: If the direction is not specified, the direction of maximum gain is assumed.
Effective isotropically-radiated power (EIRP):