Equivalent isotropically radiated power
In radio communication systems, equivalent isotropically radiated power (EIRP) or, alternatively, effective isotropically radiated power is the amount of power that a theoretical isotropic antenna (which evenly distributes power in all directions) would emit to produce the peak power density observed in the direction of maximum antenna gain. EIRP can take into account the losses in transmission line and connectors and includes the gain of the antenna. The EIRP is often stated in terms of decibels over a reference power emitted by an isotropic radiator with an equivalent signal strength. The EIRP allows comparisons between different emitters regardless of type, size or form. From the EIRP, and with knowledge of a real antenna's gain, it is possible to calculate real power and field strength values.
This example uses dBm, although it is also common to see dBW.
Decibels are a convenient way to express the ratio between two quantities. dBm uses a reference of 1 mW and dBW uses a reference of 1 W.
A transmission output of 50 W is the same as 17 dBW or 47 dBm.
The EIRP is used to estimate the service area of the transmitter, and to coordinate transmitters on the same frequency so that their coverage areas do not overlap.
In built-up areas, regulations may restrict the EIRP of a transmitter to prevent exposure of personnel to high power electromagnetic fields, however EIRP is normally restricted to minimise interference to services on similar frequencies.
See also 
- Effective radiated power, which is similar to EIRP but may use some other reference antenna than an isotropic antenna; e.g., a half-wave dipole
- Effective monopole-radiated power, which is similar to EIRP but with a reference antenna of an electrically short monopole
- IEEE Std. 100 The Authoritative Dictionary of IEEE Standards Terms, 7th Edition, The Institute of Electrical and Electronics Engineers, New York, 2000, ISBN 0-7381-2601-2, page 391
- Recommendation ITU-R BS.561-2, Definitions of radiation in LF, MF and HF broadcasting bands