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Part 15 limits check
Field strength, power and antenna limits for unlicensed AM and FM, and a quick check of your transmitter.
FM: 250 microvolts per meter, measured at 3 meters
On FM (88-108 MHz), an unlicensed transmitter has to stay inside a 200 kHz channel, with field strength no greater than 250 µV/m (48 dBµV/m) at a distance of 3 meters (47 CFR 15.239). That works out to roughly 18.75 nanowatts radiated in free space.
AM: 100 milliwatts with a 3-meter antenna
On AM (510-1705 kHz), an unlicensed transmitter is limited to 100 mW of input to the final RF stage, and the antenna, ground lead and transmission line combined cannot exceed 3 meters (47 CFR 15.219). Anything emitted outside the AM band has to be attenuated by 20 dB or more.
More Part 15 choices for AM
- Carrier current and leaky coax: on 525-1705 kHz, field strength is capped at 15 µV/m, taken from the cable or power line at a distance in meters of 47,715 / frequency-in-kHz (roughly 48 m at 1000 kHz). 47 CFR 15.221(a).
- Campus AM: on its own campus, a school or college station has no field strength cap, provided the general limit is met at the campus boundary (24 µV/m at 30 m for 1000 kHz) and keeps out of the protected contours of licensed AM stations. 47 CFR 15.221(b), (d) and 15.209.
All the rules together
| Service | Limit | Rule |
|---|---|---|
| FM, 88-108 MHz | 250 µV/m at 3 m in a 200 kHz channel; the 15.209 general limits outside it | 15.239 |
| AM, 510-1705 kHz | 100 mW input; 3 m line, antenna and ground lead | 15.219 |
| AM carrier current | 15 µV/m at 47,715 / f(kHz) m | 15.221 |
| Equipment | FCC-certified before it is sold (carrier current and campus AM: supplier's declaration) | 15.201 |
| Interference | No harmful interference; stop if you cause it | 15.5 |
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Based on the FCC rules in the eCFR as of October 14, 2026. This is guidance, not legal advice. Not an FCC product.