Effective radiated power
In electromagnetic transmission systems, particularly those dealing with telecommunications, effective radiated power or equivalent radiated power (ERP) is a standardized theoretical measurement of radio frequency (RF) energy using the SI unit watt, and is determined by subtracting system losses and adding system gains. ERP takes into consideration transmitter power output (TPO), transmission line attenuation (electrical resistance and RF radiation), RF connector insertion losses and antenna gain (the combination of antenna efficiency and antenna directivity), but not height above average terrain (HAAT). ERP is typically applied to antenna systems.
For a simplified case, if an antenna system has 9 dBd gain and 6 dB loss, its ERP is 3 dB more than (equal to double) the TPO. The use of circular polarization, or otherwise splitting equally between horizontal and vertical linear polarization, causes a "loss" of 3 dB, cutting the reported ERP in half. If full-wavelength spacing is used between antenna elements in an array, the ERP is increased approximately in proportion to the number of elements. When half-wave-spaced elements are used (to reduce RF radiation beneath the radio tower, or for other purposes), the increase is less. Null fill also detracts from the ERP by increasing the power in the nulls that form between side lobes. Stations with beam tilt often have two ERPs listed: one at the angle of tilt (the main lobe), and another in the standard horizontal plane. When only one ERP is listed, this is usually referring to the power in the main lobe.
For example, an FM radio station which advertises that it has 100,000 watts of power actually has 100,000 watts ERP, and not an actual 100,000-watt transmitter. The TPO of such a station typically may be 10,000 to 20,000 watts, with a gain factor of 5 to 10 (5× to 10×, or 7 to 10 dB). In most antenna designs, gain is realized primarily by concentrating power toward the horizontal plane and suppressing it at upward and downward angles, through the use of phased arrays of antenna elements. The distribution of power versus elevation angle is known as the vertical pattern. When an antenna is also directional horizontally, gain and ERP will vary with azimuth (compass direction). Rather than the average power over all directions, it is the apparent power in the direction of the antenna's main lobe that is quoted as a station's ERP (this statement is just another way of stating the definition of ERP). This is particularly applicable to the huge ERPs reported for shortwave broadcasting stations, which use very narrow beam widths to get their signals across continents and oceans.
United States regulatory usage
ERP for FM radio in the United States is always relative to a theoretical reference half-wave dipole antenna. (That is, when calculating ERP, the most direct approach is to work with antenna gain in dBd). To deal with antenna polarization, the Federal Communications Commission (FCC) lists ERP in both the horizontal and vertical measurements for FM and TV. Horizontal is the standard for both, but if the vertical ERP is larger it will be used instead.
The maximum ERP for US FM broadcasting is usually 100,000 watts (FM Zone II) or 50,000 watts (in the generally more densely populated Zones I and I-A), though exact restrictions vary depending on the class of license. Some stations have been grandfathered in or, very infrequently, been given a waiver, 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 used as a reference antenna, and then one speaks of EIRP (effective isotropic radiated power) rather than ERP. This includes satellite transponders, radar, and other systems which use microwave dishes and reflectors rather than dipole-style antennas.
In the case of mediumwave (AM) stations in the United States, power limits are set to the actual transmitter power output, and ERP is not used in normal calculations. Omni-directional antennas used by a number of stations radiate the signal equally in all directions. Directional arrays are used to protect co- or adjacent channel stations, usually at night, but some run directionally 24 hours. While antenna efficiency and ground conductivity are taken into account when designing such an array, the FCC database shows the station's transmitter power output, not ERP.
- Effective monopole radiated power (EMRP) may be used in Europe, particularly in relation to mediumwave broadcasting antennas. This is the same as ERP, except that a short vertical antenna (i.e. a short monopole) is used as the reference antenna instead of a half-wave dipole.
The height above average terrain for VHF and higher frequencies is extremely important when considering ERP, as the signal coverage (broadcast range) produced by a given ERP dramatically increases with antenna height. Because of this, it is possible for a station of only a few hundred watts ERP to cover more area than a station of a few thousand watts ERP, if its signal travels above obstructions on the ground.