EIRP vs ERP: What That 2.15 dB Actually Means
EIRP and ERP are the same number with two different reference antennas. Here's why the 2.15 dB conversion exists and when each one is the right metric to cite.
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) − 2.15When 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. ICNIRP power-density limits don't care which you use as long as you're consistent.
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.