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What are EIRP and ERP?

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Equivalent isotropically radiated power (EIRP) is the product of the power supplied to the antenna and its gain relative to an isotropic antenna in a given direction; effective radiated power (ERP) uses gain relative to a half-wave dipole.[1] Hence EIRP(dBm) = Pt(dBm) − Lcable(dB) + G(dBi) and EIRP = ERP + 2.15 dB.

How do you compute EIRP?

Example: a 20 dBm radio, 1.5 dB of cable and connector loss and a 6 dBi antenna gives EIRP = 20 − 1.5 + 6 = 24.5 dBm (about 282 mW). The power density at distance R is S = EIRP/(4πR²) and the far-field electric field E = √(30·EIRP)/R volts per meter with EIRP in watts and R in meters.

Why do regulators specify EIRP?

Interference and RF exposure depend on radiated, not conducted, power. US Part 15 rules for 2.4 GHz digital transmission systems allow 1 W conducted with up to 6 dBi antenna gain (4 W EIRP) and require reducing conducted power for higher-gain antennas, while many European short-range rules limit EIRP or ERP directly.[2][3] See EIRP limits.

What is TRP and how does it differ from EIRP?

Total radiated power (TRP) is the integral of EIRP over the sphere divided by 4π, i.e., the total power actually radiated. EIRP is directional peak-ish data; TRP is the figure used in OTA testing of mobile devices.[4]

Frequently asked questions

What is 0 dBd in EIRP terms?

An antenna with 0 dBd gain has 2.15 dBi.

Is EIRP measured or calculated?

Either; regulators accept both calculated values and radiated measurements depending on the rule.

What is 36 dBm EIRP in watts?

About 4 W.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

  1. “Radio Regulations”. International Telecommunication Union. 2024 edition.
  2. “47 CFR 15.247 — Operation within the bands 902-928 MHz, 2400-2483.5 MHz, and 5725-5850 MHz”. U.S. FCC, eCFR. current text.
  3. “ETSI EN 300 328: Wideband transmission systems operating in the 2.4 GHz band”. ETSI. current version.
  4. “Test Plan for Wireless Device Over-the-Air Performance”. CTIA Certification. current version.
Categories: Fundamentals