# EIRP / ERP

> Transmitter power multiplied by antenna gain: the figure that, with distance, determines free-space power density.

- Canonical URL: https://emfbase.com/glossary/eirp
- Collection: Glossary
- Record id: `glossary:eirp`
- Status: verified
- Last verified: 2026-08-12
- JSON: https://emfbase.com/api/glossary/eirp

## Key values

- **Category:** technology

## Definition

Equivalent isotropically radiated power is the power an isotropic radiator would need to produce the same field on the antenna's main beam. ERP is the same idea referenced to a half-wave dipole (ERP × 1.64 = EIRP). Both describe the main beam only, so using EIRP with an inverse-square formula gives a boresight upper bound and overstates exposure everywhere off-axis — which is why levels at the base of a tower are far below the value the formula suggests.

## Sources

- `fcc-oet65` Federal Communications Commission, Office of Engineering and Technology (1997). *OET Bulletin 65: Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields*. https://www.fcc.gov/general/oet-bulletins-line — government, link checked 2026-08-12
- `itu-fs-relations` International Telecommunication Union. *Free-space path loss and power density relations (ITU-R P.525)*. https://www.itu.int/rec/R-REC-P.525/en — standard, link checked 2026-08-12

Data licence: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Dataset version 2026.08.1. Retrieved from EMFBase, https://emfbase.com.

This record is reference data, not medical or legal advice. Regulatory limits, scientific guidelines and precautionary values are labelled separately and answer different questions.