EIRP / ERP

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

1. fcc-oet65 OET Bulletin 65: Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields — Federal Communications Commission, Office of Engineering and Technology, 1997. government, link checked 2026-08-12.
2. itu-fs-relations Free-space path loss and power density relations (ITU-R P.525) — International Telecommunication Union. standard, link checked 2026-08-12.