Effective Isotropic Radiated Power (EIRP)

Effective Isotropic Radiated Power (EIRP) quantifies the total power radiated by a transmitting system in the direction of maximum antenna gain, assuming an ideal isotropic radiator. It combines the transmitter’s output power and the antenna’s directional gain to provide a standardized measure of radiated signal strength.

How EIRP Is Calculated

In linear terms:

EIRP = P_t x G_t

Where:

• P_t = Transmitter power (watts)

• G_t = Antenna gain (unitless, linear scale)

In decibel form (more common):
EIRP (dBm) = P_t (dBm) + G_t (dBi)

Including transmission losses:
EIRP (dBm) = P_t (dBm) − L_c (dB) + G_t (dBi)

Where:

• L_c = Cable and connector losses (in dB)

These equations are essential in wireless system planning, link budget analysis, and regulatory compliance.

Example: Wi-Fi Access Point

A Wi-Fi device with:

• Transmitter power: 100 mW (20 dBm)

• Antenna gain: 6 dBi

• Cable losses: 0 dB

Results in:
EIRP = 20 dBm + 6 dBi = 26 dBm
(≈ 400 mW radiated in the direction of peak antenna gain)

This illustrates how EIRP accounts for both the device's power output and antenna characteristics.

Regulatory Relevance and Applications

EIRP limits are defined by regulatory bodies such as:

• FCC (U.S.): e.g., Part 15.247 for unlicensed bands

• ETSI (Europe): e.g., EN 300 328 for the 2.4 GHz band

These limits help manage interference and ensure fair spectrum usage. Regulatory compliance testing verifies that EIRP stays within permissible boundaries for a given frequency band and application.

While EIRP is the standard reference in many applications, the FCC also uses ERP as the regulatory reference in certain domains such as broadcasting or cellular services under Part 22. Therefore, converting between EIRP and ERP may be necessary when preparing documentation for international market approval.

EIRP is used in:

• Wireless LANs and cellular networks

• Satellite and microwave communication systems

• Broadcasting and point-to-point radio links

It is also central to link budget calculations, coverage planning, and antenna system design.