In electromagnetic transmission systems, particularly those dealing with telecommunications, effective
radiated power or equivalent radiated power is a standardized theoretical measurement
of radio frequency energy using the SI unit watts, and is determined by subtracting system
losses and adding system gains.
ERP takes into consideration transmitter power output, transmission line attenuation, RF
connector insertion losses, and antenna directivity, but not height above average terrain.
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 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, 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, and another
in the standard horizontal plane.
When only one ERP is listed, this is usually referring to the power in the main lobe.
FM example 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.
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.
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 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..
To deal with antenna polarization, the Federal Communications Commission 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 or 50,000 watts, 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 is used as a reference antenna, and then one speaks of EIRP rather than ERP.
This includes satellite transponders, radar, and other systems which use microwave dishes
and reflectors rather than dipole-style antennas.
Lower-frequency issues In the case of mediumwave 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.
Related terms Effective monopole radiated power may be used
in Europe, particularly in relation to mediumwave broadcasting antennas.
This is the same as ERP, except that a short vertical antenna is used as the reference
antenna instead of a half-wave dipole.
Equivalent isotropically radiated power HAAT
The height above average terrain for VHF and higher frequencies is extremely important
when considering ERP, as the signal coverage 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 one of a few thousand, if its signal travels above obstructions on the ground.
See also Nominal power
List of broadcast station classes References