Discussion

Encyclopedia
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

(RF) energy using the SI
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

unit watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

s, and is determined by subtracting system losses and adding system gain
Antenna gain
In electromagnetics, an antenna's power gain or simply gain is a key performance figure which combines the antenna's directivity and electrical efficiency. As a transmitting antenna, the figure describes how well the antenna converts input power into radio waves headed in a specified direction...

s. ERP takes into consideration transmitter power output
Transmitter power output
In radio transmission, transmitter power output is the actual amount of power of radio frequency energy that a transmitter produces at its output....

(TPO), transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...

attenuation
Attenuation
In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...

(electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

RF connector
A coaxial RF connector is an electrical connector designed to work at radio frequencies in the multi-megahertz range.RF connectors are typically used with coaxial cables and are designed to maintain the shielding that the coaxial design offers. Better models also minimize the change in transmission...

insertion loss
Insertion loss
In telecommunications, insertion loss is the loss of signal power resulting from the insertion of a device in a transmission line or optical fiber and is usually expressed in decibels ....

es, and antenna directivity
Directional antenna
A directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources....

, but not height above average terrain
Height above average terrain
Height above average terrain is used extensively in FM radio and television, as it is actually much more important than effective radiated power in determining the range of broadcasts...

(HAAT). ERP is typically applied to antenna systems.

For a simplified example, if an antenna system has 9 dB gain and 6 dB loss, its ERP is 3 dB over (equal to double) the TPO. The use of circular polarization
Circular polarization
In electrodynamics, circular polarization of an electromagnetic wave is a polarization in which the electric field of the passing wave does not change strength but only changes direction in a rotary type manner....

, or otherwise splitting between horizontal and vertical linear polarization
Linear polarization
In electrodynamics, linear polarization or plane polarization of electromagnetic radiation is a confinement of the electric field vector or magnetic field vector to a given plane along the direction of propagation...

, causes a "loss" of 3dB, cutting the reported ERP in half. If full-wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

spacing is used between antenna elements in an array, the ERP is increased multiplicative
Multiplication
Multiplication is the mathematical operation of scaling one number by another. It is one of the four basic operations in elementary arithmetic ....

ly with the number of elements. For half-wave-spaced elements (used to limit RF radiation hazard beneath the radio tower), only half of the elements count. Null fill
Null fill
Null fill is used in radio antenna systems which are located on mountains or tall towers, to prevent too much of the signal from overshooting the nearest part of intended coverage area. Phasing is used between antenna elements to take power away from the main lobe and electrically direct more of...

also detracts from the ERP by increasing the power in the nulls that form between side lobe
Side lobe
In antenna engineering, side lobes or sidelobes are the lobes of the far field radiation pattern that are not the main lobe....

s. Stations with beam tilt
Beam tilt
Beam tilt is used in radio to aim the main lobe of the vertical plane radiation pattern of an antenna below the horizontal plane.The simplest way is mechanical beam tilt, where the antenna is physically mounted in such a manner as to lower the angle of the signal on one side...

often have two ERPs listed: one at the angle of tilt (the main lobe
Main lobe
The main lobe, or main beam, of an antenna radiation pattern is the lobe containing the maximum power. This is the lobe that exhibits the greatest field strength....

), and another in the standard horizontal plane
Horizontal plane
In geometry, physics, astronomy, geography, and related sciences, a plane is said to be horizontal at a given point if it is perpendicular to the gradient of the gravity field at that point— in other words, if apparent gravity makes a plumb bob hang perpendicular to the plane at that point.In...

. When only one ERP is listed, this is usually referring to the power in the main lobe.

## FM example

For example, an FM
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...

which advertises that it has 100,000 watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

s of power actually has 100,000 watts ERP, and probably 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 10dB
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...

). In most antenna designs, gain is realized primarily by concentrating power toward the horizontal plane
Horizontal plane
In geometry, physics, astronomy, geography, and related sciences, a plane is said to be horizontal at a given point if it is perpendicular to the gradient of the gravity field at that point— in other words, if apparent gravity makes a plumb bob hang perpendicular to the plane at that point.In...

and suppressing it at upward and downward angles, through the use of phased array
Phased array
In wave theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.An antenna array...

s 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
Compass
A compass is a navigational instrument that shows directions in a frame of reference that is stationary relative to the surface of the earth. The frame of reference defines the four cardinal directions – north, south, east, and west. Intermediate directions are also defined...

direction). Rather than the average
Average
In mathematics, an average, or central tendency of a data set is a measure of the "middle" value of the data set. Average is one form of central tendency. Not all central tendencies should be considered definitions of average....

, it is the maximum ERP in any direction that is usually quoted as a station's power. This is particularly applicable to the huge ERPs reported for shortwave broadcasting stations, which use very narrow beam widths to get their signals across continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...

s and ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

s.

### 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
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...

(FCC) lists ERP in both the horizontal and vertical measurement
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

s 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 (the more densely populated FM zones I/I-A), though exact restrictions vary depending on the class of license. Some stations have been grandfathered
Grandfather clause
Grandfather clause is a legal term used to describe a situation in which an old rule continues to apply to some existing situations, while a new rule will apply to all future situations. It is often used as a verb: to grandfather means to grant such an exemption...

in or, very infrequently, been given a waiver
Waiver
A waiver is the voluntary relinquishment or surrender of some known right or privilege.While a waiver is often in writing, sometimes a person's actions can act as a waiver. An example of a written waiver is a disclaimer, which becomes a waiver when accepted...

, and can exceed normal restrictions.

## Microwave band issues

For most microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

systems, a completely non-directional isotropic antenna (one which radiate

s equally and perfectly well in every direction – a physical impossibility) is used as a reference antenna. This includes satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

transponder
Transponder
In telecommunication, the term transponder has the following meanings:...

s, radar, and other systems which use microwave dishes and reflectors rather than dipole-style antennas.

## Lower-frequency issues

In the case of mediumwave
Mediumwave
Medium wave is the part of the medium frequency radio band used mainly for AM radio broadcasting. For Europe the MW band ranges from 526.5 kHz to 1606.5 kHz...

(AM) stations in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, actual radiated power is used for an omnidirectional
Omnidirectional
Omnidirectional refers to the notion of existing in every direction. Omnidirectional devices include:* Omnidirectional antenna, an antenna that radiates equally in all directions* VHF omnidirectional range, a type of radio navigation system for aircraft...

station; for a directional station, power is computed relative to an omnidirectional radiator with the same nominal power
Nominal power
Nominal power is a measurement of a mediumwave radio station's output used in the United States. AM broadcasters are licensed by the Federal Communications Commission to operate at a specific nominal power, which may be different from the transmitter power output.* For non-directional stations,...

and an efficiency equal either to the RMS
Root mean square
In mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids...

efficiency of the directional antenna under consideration, or to the minimum efficiency permitted for the class of station.

## Related terms

• Effective monopole radiated power (EMRP) may be used in Europe, especially in relation to mediumwave
Mediumwave
Medium wave is the part of the medium frequency radio band used mainly for AM radio broadcasting. For Europe the MW band ranges from 526.5 kHz to 1606.5 kHz...

broadcasting antennas. This is the same as ERP, except that a short vertical antenna (i.e. a short monopole
Monopole
Monopole may refer to:*Magnetic monopole, or Dirac monopole, a hypothetical particle that may be loosely described as a magnet with only one pole, or related concepts in physics and mathematics:...

) is used as the reference antenna instead of a half-wave dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...

.