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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) power〔(【引用サイトリンク】 publisher = Institute for Telecommunication Sciences )〕〔(【引用サイトリンク】 publisher = International Telecommunication Union )〕 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.
== 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 (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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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