Effective Radiated Power is the product of the power supplied to the antenna and the gain of the antenna, relative to a reference antenna, in agiven direction. So the ERP is the power that would be necessary at the input of the reference antenna to give the same amount of radiated power in the same direction. Usually, the antenne gain is relative to a half-wave dipole antenna.

If an isotropic antenna is used as reference antenna the term EIRP is used.

Because a haf wavelength dipole antenna has a antenna gain, relative to isotropic radiation. The ERP is smaller than the EIRP. The difference is:

ERP = EIRP/1,64 = EIRPdB - 2,15 dB.

See also