e.r.p. Output Power (Effective Radiated Power)
The quantity expressing the power a transmitter actually radiates into the air including antenna gain; it is defined relative to a half-wave dipole antenna. For short range devices the limits are given in terms of this quantity: 10 mW for 433.050 - 434.790 MHz, 25 mW for the 868 MHz sub-bands.
What matters is that the limit applies to the power radiated from the antenna, not the power at the output of the transmitter circuit. As antenna gain increases, the radiated power rises for the same circuit power; that is why changing the antenna directly concerns the regulatory limit.
- The milliwatt order is not misleading: hundreds of metres of range can be achieved outdoors with 10 mW, because what determines range is receiver sensitivity and the environment.
- Doubling the power does not double the range; in free space distance scales roughly with the square root of power.
- The limit varies per band and is assessed together with the duty cycle restriction.
The practical conclusion is this: in a range problem the greatest gain is not in power but in antenna placement. Moving the antenna outside the metal enclosure, positioning it vertically and keeping it a few centimetres away from large metal surfaces improves range noticeably without exceeding the permitted power limit. Shortening or lengthening the antenna cable, on the other hand, upsets length and impedance matching.
Context in which this term is used: Wireless Crane Remote Control Selection Guide