What is the difference between ERP and EIRP?
ERP is radiated power referenced to a half-wave dipole, EIRP is referenced to an isotropic antenna, and the fixed difference between them is 2.15 dB: EIRP(dBm) = ERP(dBm) + 2.15. So 10 mW ERP is roughly 16.4 mW EIRP, and 25 mW ERP is roughly 41 mW EIRP. Sub-GHz SRD limits are generally given as ERP, while 2.4 GHz limits (20 dBm) are given as EIRP.
Both answer the question "how much power actually leaves the antenna"; the only difference is which reference antenna the measurement is against.
The calculation chain is this: ERP = Transmitter power − feed loss + antenna gain (dBd). Do the same calculation in dBi and you get EIRP. Because a dipole has 2.15 dBi of gain relative to an isotropic antenna, the difference between the two results is always 2.15 dB.
Why does this matter? Because the regulatory limits are given in mixed units:
- The sub-GHz limits in ERC/REC 70-03 (10 mW at 433 MHz, 25 mW at 863–870 MHz, 500 mW at 869.4–869.65 MHz) are in ERP.
- The 2.4 GHz limit of 20 dBm in EN 300 328 is in EIRP.
Confusing the two produces a systematic error of 2.15 dB. In a conformity measurement, that can decide whether you pass the limit or not. In design it leads to this conclusion: when the limit is given as ERP, you cannot gain range by raising antenna gain — you have to cut the transmitter output instead; what is limited is not the chip output but the total power put into the air.
Full topic: Wireless remote control installation: pairing, relay, safety