EIRP vs ERP: What That 2.15 dB Actually Means

One number, two references#

EIRP (Effective Isotropic Radiated Power) references an ideal isotropic radiator. ERP (Effective Radiated Power) references a half-wave dipole. The dipole has 2.15 dBi of gain over isotropic, so:

ERP (dBm) = EIRP (dBm) − 2.15

When to use which#

FCC limits for broadcast services are typically expressed in ERP. FCC OET rules for cellular and microwave links almost always use EIRP. ITU and most international RF safety analysis use EIRP.

Quick check#

If a datasheet gives antenna gain in dBd (decibels relative to a dipole), add 2.15 to get dBi. The EIRP calculator handles both forms.