Effective radiated power

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In radio telecommunications, effective radiated power or ERP is determined by subtracting system losses from system gains. ERP is typically applied to antenna systems. For example, if an antenna system has +9dB gain and −6dB loss, its ERP is +3dB over the transmitter power output (TPO).

For example, an FM radio station which advertises that it has 100,000 watts of power actually has 100,000 watts ERP, and not a 100,000-watt transmitter. The TPO of such a station is most likely around 10,000-20,000 watts, with a gain of +7 to +10dB (5× to 10×). In addition, if the antenna is directional, the ERP figure is usually the maximum in any one direction, rather than the average.

ERP is always relative to a reference antenna. A dipole antenna is most often used, but an imaginary isotropic antenna which radiates equally in all directions may also be used.

From FS-1037C:

Effective radiated power:

1. The power supplied to an antenna multiplied by the antenna gain in a given direction.

• Note 1: If the direction is not specified, the direction of maximum gain is assumed.
• Note 2: The type of reference antenna must be specified.

2. The product of the power supplied to the antenna and its gain relative to a half-wave dipole in a given direction (used by the NTIA and FCC).

• Note: If the direction is not specified, the direction of maximum gain is assumed.

Effective isotropically-radiated power (EIRP):

The arithmetic product of (a) the power supplied to an antenna and (b) its gain relative to an isotropic source.