In radio communication systems, Effective isotropically-radiated power (EIRP) is the amount of power that would have to be emitted by an isotropic antenna (that evenly distributes power in all directions) to produce the peak power density observed in the direction of maximum antenna gain. EIRP takes into account the losses in transmission line and connectors and the gain of the antenna. The EIRP is often stated in terms of decibels over a reference power level, that would be the power emitted by an isotropic radiator with an equivalent signal strength.

EIRP(dBm) = (power of transmitter (dBm)) – (losses in transmition line (dB)) + (antenna gain(dB))

where antenna gain is expressed relative to a (theoretical) isotropic reference antenna.

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 1mW and dBw uses a reference of 1W.

dBm = 10 log(power out / 1mW)

and

dBw = 10 log(power out / 1W)

A transmitter with a 50W output can be expressed as a 16dBw output

16.9897 = 10 * log(50/1)

The EIRP is used to estimate the service area of the transmitter, and to co-ordinate 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.

See also

• Effective radiated power