edVirtus Reference

What Are Effective Radiated Power and Effective Isotropic Radiated Power?

How Do ERP and EIRP Describe Directional Radiated Power?

Effective radiated power (ERP) and effective isotropic radiated power (EIRP) describe the directional strength of a transmitting system. Each combines the power delivered toward the antenna, losses between the transmitter and antenna, and antenna gain into an equivalent radiated-power quantity.

Neither quantity is simply the transmitter's rated output. Power may be lost in cables, waveguides, connectors, filters, and switches before reaching the antenna. The antenna then redistributes the remaining power according to its radiation pattern, concentrating more energy in some directions than others.

ERP and EIRP differ only in their reference antenna. ERP uses an ideal half-wave dipole, and antenna gain is expressed in dBd. EIRP uses an ideal isotropic radiator, and gain is expressed in dBi. An isotropic radiator is a theoretical point reference that radiates equally in every direction.

EIRP (dBW) = transmitter power (dBW) - feeder loss (dB) + antenna gain (dBi)

ERP (dBW) = transmitter power (dBW) - feeder loss (dB) + antenna gain (dBd)

A half-wave dipole has a maximum gain of approximately 2.15 dB relative to an isotropic radiator. For the same transmitting system, EIRP is therefore 2.15 dB greater than ERP. In linear units, EIRP is approximately 1.64 times ERP, and ERP is approximately 0.61 times EIRP.

For example, a system with 10 W of transmitter power, 2 dB of feeder loss, and 20 dBi of antenna gain has an EIRP of 28 dBW, or about 631 W. Its corresponding ERP is 25.85 dBW, or about 385 W. No additional power has been created; the antenna has concentrated the available power directionally.

ERP and EIRP normally refer to the direction of maximum antenna gain unless another direction is specified. They should not be interpreted as the total power radiated in every direction. Total radiated power is obtained by integrating radiation over the full antenna pattern and is a different quantity.

These equivalent powers are useful inputs to far-field calculations because they combine source power and directional antenna gain. In an unobstructed far-field direction, power density can often be estimated from EIRP and distance, subject to the assumptions of the calculation.

ERP or EIRP alone does not determine human exposure. Exposure also depends on frequency, antenna pattern and beam direction, duty cycle, distance, access, reflections, shielding, and the person's location. In the near field or close to the body, a simple EIRP-based far-field calculation may be unsuitable.

Communication engineers use EIRP in satellite links, microwave systems, radar, broadcasting, cellular networks, and wireless local-area networks. It is a central link-budget quantity because it expresses the signal strength launched toward a receiver after feeder loss and antenna gain are considered.

ERP remains common in broadcasting and in regulations or equipment specifications that use a half-wave dipole reference. EIRP is common where isotropic-reference gain is standard. Regulatory limits may specify either quantity, so the stated reference must always be checked before values are compared.

A reported ERP or EIRP value should identify the averaging basis, modulation or operating condition, feeder losses, antenna gain reference, direction, units, and whether the value is a maximum, nominal, or time-averaged quantity. Values expressed in watts, dBW, or dBm must be converted consistently.

Transmitter output power, ERP, EIRP, total radiated power, and incident power density are related but not interchangeable. Treating one as another can produce large errors in link calculations and RF exposure assessments.

ERP and EIRP therefore provide compact, useful descriptions of directional radiated strength. Their value lies in combining transmitter power, feeder loss, and antenna gain while retaining a clearly stated reference antenna.