ISM Bands Explained
Edit on GitHubWhat an ISM band is, the 433 MHz allocation, and why "unlicensed" still means "licensed-free within limits".
ISM Bands Explained
The beacon transmits in the 433 MHz ISM band. ISM stands for industrial, scientific, and medical, and the name explains both the freedom and the limits.
What ISM means
ISM bands are frequency ranges reserved for equipment that is not primarily a radio service: microwave ovens (2.4 GHz), medical diathermy, and industrial heating. Because the primary users are allowed to radiate, everything else shares the band under strict limits.
The 433 MHz allocation
In Europe and much of the world, 433.05-434.79 MHz is an ISM allocation. Devices may operate without an individual license, but typically at 10 mW ERP maximum and often with duty-cycle restrictions.
The catch for the beacon
The beacon transmits at 17-22 dBm (50-160 mW), well above the 10 mW ERP limit. Operating at those levels is generally not “unlicensed” in most jurisdictions:
- With a ham license, you may operate at higher power on the amateur allocations (70 cm band: 430-440 MHz in Europe).
- Some countries have emergency-use exceptions for genuine life-threatening situations.
- PMR446 (446 MHz) is license-free at 500 mW ERP in the EU, and the firmware supports those frequencies.
The practical advice
- For testing and bench work, keep power low and use a dummy load.
- For legal unlicensed operation, use PMR446 frequencies within their limits.
- For full-power emergency use, hold a ham license and check the local band plan.
- Never transmit on frequencies used by safety services.
Where to check your rules
The frequency compatibility page has a regional table. The authoritative source is always your national regulator: FCC (US), Ofcom (UK), ANFR (France), BNetzA (Germany), and their equivalents.
The design choice
The beacon can transmit within legal limits; it also can go to +22 dBm. The firmware makes the power configurable precisely so you can match your local rules. The default of +17 dBm is a compromise between range and the power most operators can legally use with a license.