Effective Isotropic Radiated Power (EIRP)
Effective Isotropic Radiated Power (EIRP) is the output power when a signal is concentrated into a smaller area by the Antenna.
An isotropic radiator radiates power equally in all directions, however a perfect isotropic radiator is only theoretical as even the simplest antennas will concentrate the signal in certain direction(s). E.g. a 1/2 wave dipole has a gain of 2.15 dBi.
The EIRP is calculated using this formula:
EIRP = Effective Isotropic Radiated Power
Pout = transmitter power output (dBm)
Ct = signal loss in cable (dB)
Gt = gain of the antenna (dBi)
Pout – Ct + Gt = EIRP
When installing a wireless system with external antenna, your EIRP calculation should not exceed the class license limit. Other wise you must adjust either the transmitter power output, the length of cable and/or the choice of antenna.