Effective radiated power
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In radio telecommunications, effective radiated power or equivalent radiated power (ERP) is a standardized theoretical measurement of radio frequency (RF) energy[1] [2] using the non-SI unit dBd, and is determined by subtracting system losses and adding system gains. ERP takes into consideration transmitter power output (TPO), transmission line losses, connector losses, antenna directivity, and height above average terrain (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 the TPO.
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[edit] 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 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×). In most antenna designs, gain is realized primarily by concentrating power toward the horizon and suppressing it at upward and downward angles. 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. The maximum ERP is usually quoted.
[edit] 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 (FCC) lists ERP in both the horizontal and vertical planes.
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 in or, very infrequently, been given special dispensation, and can exceed normal restrictions.
[edit] 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. This includes satellite transponders, radar, and other systems which use microwave dishes and reflectors rather than dipole-style antennas.
[edit] 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.
[edit] Related terms
- Effective monopole radiated power (EMRP) may be used in Europe, especially 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.
[edit] HAAT
The height above average terrain for VHF and higher frequencies is extremely important when specifying ERP, as the signal coverage (range) produced by a given ERP dramatically increases with antenna height.
[edit] See also
[edit] References
- ^ "Abbreviations and Acronyms". Institute for Telecommunication Sciences. http://www.its.bldrdoc.gov/fs-1037/dir-013/_1908.htm. Retrieved on 2008-08-29.
- ^ "Recommendation ITU-R M.1224". International Telecommunication Union. http://www.itu.int/ITU-R/asp/terminology-definition.asp?lang=en&rlink={366E7A63-5216-404F-B7F9-EAFA78CB464D}. Retrieved on 2008-08-29.
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