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Effective Isotropic Radiated Power (EIRP)
Effective isotropic radiated power (sometimes also referred to as equivalent isotropic radiated power), is a commonly used unit when specifying antenna efficiency (gain).
In order to provide a common reference for radiated power, an ideal isotropic radiator is used as the standard. An isotropic radiator emits power from a singular point (dimensionless) whose wavefront is a perfect sphere of constant voltage (or power for equal impedances). Any gain specified for an antenna represents a concentration of the radiation pattern in a given direction. EIRP is calculated as follows: