ERP (Effective Radiated Power)
ERP expresses the effective power radiated by a system referenced to a half-wave dipole, and it is found by subtracting feed losses from the transmitter power and adding the antenna gain (dBd). Most short range device limits in Europe are written in this unit; for example, under the CEPT framework the typical limit for the 433 MHz band is 10 mW e.r.p.
ERP matters for understanding correctly what the regulation actually limits: what is limited is not the watt figure at the transmitter output, but the effective power genuinely radiated into the air.
- Calculation: ERP(dBm) = transmitter output power (dBm) − feed/cable loss (dB) + antenna gain (dBd).
- Consequence: fitting a higher gain antenna can push ERP above the limit without changing the transmitter power at all.
- Conversion: EIRP(dBm) = ERP(dBm) + 2.15.
This explains why swapping the antenna in the field is not a free action. A device's declaration of conformity is based on measurements made with a specific antenna and a specific configuration. Tampering with the antenna does not only add range, it can take the device outside the declared conformity. AXI sets ship the receiver with its own antenna and cable glands; where an antenna change is needed, consulting the manufacturer is the right course.
Context where this term is used: Wireless remote control installation: pairing, relay, safety