How close is too close?
The FCC requires every ham to check that nobody gets too much RF from their antenna. Answer a few questions and this page does the check for you. How it works
Which one do I pick?
- Talking on SSB
- Your voice has gaps between words and syllables, so 100 W of SSB averages about 20 W. That is the first option. A speech processor or compressor fills the gaps in: pick 50%.
- FM, on a repeater or simplex
- A full-power carrier the entire time you hold the button, whether or not you are talking. 100%.
- CW
- The key is down for dits and dahs and up between them. Normal sending works out to about 40%.
- FT8, FT4, WSPR, JS8, MSK144
- A steady tone at full power for the whole transmit period: 100%, no exceptions. They do listen a lot, though. FT8 and FT4 take turns in equal slots, so the pattern is one minute on, one off. WSPR at its default 20% transmit rate is about two minutes on, eight off. JS8 and MSK144 vary; leave the pattern at worst case unless you know yours.
- RTTY, PSK31, Olivia, packet, Winlink, VARA, SSTV
- Anything the computer sends is full power from start to finish. 100%.
- DMR, D-STAR, Fusion, P25
- Digital voice keeps transmitting through your pauses. DMR splits the channel into two time slots, but treat every digital voice mode as 100% unless you can show otherwise.
- Not sure
- Pick 100%. The only mistake you can make with the duty factor is being too generous to yourself.
Show the numbers
| PUBLIC 30 MIN AVG | HOUSEHOLD 6 MIN AVG | |
|---|---|---|
| Limit (mW/cm²) | — | — |
| Averaged power (W) | — | — |
| EIRP (W) | — | — |
| Density at nearest person | — | — |
| Safe distance | — | — |
HOW THIS WORKS, AND WHERE THE RULES COME FROM
Radio energy spreads out
Double the distance and the energy hitting you drops to a quarter: S = EIRP / 4πR². A wave bouncing off the ground can add to the direct one, which is the 2.56× allowance. This is the method in FCC OET Bulletin 65 and its amateur supplement, first put in calculator form by Wayne Overbeck N6NB in 1997.
It assumes you are in the antenna's strongest direction. That is conservative for most people, but a beam pointed at the house is exactly what it says.
Household vs the public
The FCC allows more exposure for people who know about it and can act on it: you, and household members you have briefed. It calls that a controlled environment, averaged over 6 minutes. Everyone else is uncontrolled, averaged over 30 minutes with a limit five times stricter. Both come from 47 CFR §1.1310 and depend on frequency; the body absorbs 30–300 MHz best, so the limits are tightest there.
When you can skip the paperwork
Since 2021 there is no "under 50 W" free pass. Instead, §1.1307(b)(3) exempts a station whose averaged ERP is under a threshold that grows with distance to the nearest person, once that person is beyond λ/2π. Exempt or not, the limits still apply. Exempt just means you do not have to show your work. You already did.
SOURCES · 47 CFR §1.1310 (limits) · §1.1307(b)(3) (exemption) · §97.13(c) (amateur obligation) · OET Bulletin 65 & Supplement B · typical antenna gains from the RSGB EMF exposure guidance · method after Wayne Overbeck N6NB, CQ VHF, Jan 1997.
