EIRP

Equivalent isotropically radiated power (EIRP), or synonymous Effective Isotropically Radiated Power, is an IEEE standardized definition of directional radio frequency (RF) power, in terms of the power that would be required to transmit a signal equally in all directions, from a strictly theoretical spherically radiating source. It is the total power that a distant receiver in the beam of the antenna's strongest signal would conclude was being radiated in all directions, knowing the distance to the transmitting antenna but incorrectly believing the transmitter was omnidirectional.

EIRP is differentiated from effective (or equivalent) radiated power (ERP) by use of absolute antenna gain in the calculation instead of relative antenna gain. The term "gain" is assumed to mean "antenna gain" and also "absolute" (referenced to isotropic) unless specifically stated to be relative. The gain is then multiplied by the power actually accepted by the antenna to result in the actual EIRP. Power losses which occur prior to the antenna, e.g., in the transmission line or from inefficiency in the generator itself are therefore not included in the calculation.

Antenna gain (or simply gain) is closely related to directivity (the term directive gain is deprecated by IEEE) and often incorrectly used interchangeably. However, antenna gain is always less than directivity by a factor called radiation efficiency, η. Whereas directivity is entirely a function of wavelength and the geometry and type of antenna, gain takes into account the losses that always occur in the real world. Specifically, accelerating charge (time-varying current) causes electromagnetic radiation per Maxwell's equations. Therefore, antennas use a current distribution on radiating elements to generate electromagnetic energy that propagates away from the antenna. This coupling is never 100% efficient (by Laws of Thermodynamics), and therefore gain will always be less than directivity by this efficiency factor.

An ideal isotropic radiator is a theoretical device that cannot actually exist but that provides a mathematical construct for a common baseline of comparison. Isotropic radiation is at identical power in all directions spherically from the isotropic source. In other words, a notional receiver in a given direction from the transmitter would receive the same power if the source were replaced with an isotropic source and with an antenna input power equal to the EIRP. The receiver would not be able to determine a difference.

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