In electromagnetic transmission devices, especially 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 watts, 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 directivity, but not height above average terrain (HAAT). ERP is typically applied to antenna systems.
For a simplified example, 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.
Read more about Effective Radiated Power: FM Example, Microwave Band Issues, Lower-frequency Issues, Related Terms, HAAT
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