ERP vs EIRP

Regulators quote power limits in either ERP or EIRP. Mixing them up can make you think you are legal when you are not.

The definitions

• ERP (effective radiated power): the power a half-wave dipole would need to produce the same field. It ignores the gain of a simple antenna beyond dipole equivalence.
• EIRP (effective isotropic radiated power): the power an isotropic (perfectly spherical) radiator would need. It includes the antenna gain over isotropic.

The difference is 2.15 dB: EIRP = ERP + 2.15 dB (for a dipole compared to isotropic).

Which limits use which

• EU 433 MHz ISM: often quoted as 10 mW ERP.
• PMR446: 500 mW ERP.
• Amateur: usually no ERP limit; power is quoted at the transmitter output.
• FCC: often uses EIRP.

The antenna gain trap

If the limit is 10 mW ERP and your antenna has 3 dBi of gain, the transmitter output must be reduced so that ERP stays under the limit. A high-gain antenna does not make you more legal; it reduces the allowed transmitter power.

For the beacon:

• The default whip antenna has roughly 0 dBi gain (unity).
• A ground-plane setup adds ~3 dBi, which counts against ERP/EIRP limits.

Working the numbers

At +17 dBm (50 mW) with a unity antenna:

• ERP = 50 mW × (0.75 for the mismatch losses) ≈ 37 mW ≈ +15.7 dBm ERP - still above the 10 mW ISM limit.
• To meet 10 mW ERP you would need to transmit at about +7 dBm with losses.

The practical takeaway

If your regulator says “10 mW ERP”, the beacon’s +17 dBm default is not compliant without a license. Use the dashboard’s power slider to test below the limit, or operate under a license. The legal pages and the regional frequency table give the numbers per country.