Who has to do this
Before you transmit from any place where people could be exposed above the FCC limits, you must make sure the station complies (47 CFR 97.13(c)). People who aren't in your household are evaluated against the general population limits. You may use the higher controlled limits for yourself and your household members, if you and they have the appropriate training and information (97.13(c)(1)). If an evaluation shows the limits could be exceeded where people can go, you must act to prevent that exposure (97.13(c)(2)).
How it works
- Average power. PEP times the mode's duty factor, times the share of the averaging period you're actually transmitting: 6 minutes for controlled exposure, 30 minutes for the general population (47 CFR 1.1310, Table 1).
- EIRP. Subtract feed line loss and add antenna gain (both in dB).
- Distance. R = √(0.64 × EIRP ÷ (π × S)), where S is the limit in mW/cm² at your frequency. The 0.64 includes the EPA ground-reflection factor of 2.56 (FCC OET Bulletin 65, Equation 7).
- Exemption (optional). If you enter a distance, the tool also checks the routine-evaluation exemption in 47 CFR 1.1307(b)(3)(i)(C): time-averaged ERP (EIRP ÷ 1.64) against the Table 1 threshold for your frequency. That exemption only applies at λ/2π or more from the antenna.
Duty factors the FCC lists for amateurs
| Mode | Duty factor |
|---|---|
| Conversational SSB (no speech processing) | 20% |
| Conversational SSB (heavy speech processing) | 50% |
| Voice FM, FSK/RTTY, AFSK SSB | 100% |
| Conversational CW | 40% |
| Carrier (tune-up) | 100% |
Source: OET Bulletin 65 Supplement B, Table 2.
Worked examples
- 14.35 MHz, 100 W, 0 dBi, 100% duty: general population limit 180/14.35² = 0.874 mW/cm², so R = √(0.64 × 100,000 ÷ (π × 0.874)) = 153 cm, about 1.5 m. Controlled limit 4.37 mW/cm² gives 0.7 m. These match the FCC's Supplement B Table 4a.
- 1,500 W PEP SSB, no processing, 2 minutes on and 2 off: 1,500 × 0.2 × 4/6 = 200 W average for controlled exposure, and 1,500 × 0.2 × 20/30 = 200 W for the general population (the FCC's own Supplement B example).
- 146 MHz, 2 m from the antenna: the exemption threshold is 3.83 × 2² = 15.3 W ERP. A 100 W FM signal into a 0 dBi antenna is 61 W ERP, so it isn't exempt and needs the evaluation.
All three are automated test cases for this calculator.
Common questions
Do I need to do an RF exposure evaluation?
Yes, unless your station qualifies for an exemption under 47 CFR 1.1307(b)(3). Exemption only means you don't have to do the routine evaluation; you still have to keep exposure within the limits.
Which distance do I use for my neighbors?
The general population (uncontrolled) distance. The controlled distance is only for you and trained household members (47 CFR 97.13(c)(1)).
Why is the result so conservative close to my HF antenna?
Within roughly a wavelength of the antenna, the far-field formula over-predicts power density (OET Bulletin 65). The result is a worst case; antenna modeling or measurement can show a shorter distance.
Related: coax loss calculator (for the feed line loss input), can I transmit here? and the rules we cite.
Sources
- 47 CFR 97.13 Restrictions on station location
- 47 CFR 1.1310 Radiofrequency radiation exposure limits
- 47 CFR 1.1307(b) RF exposure exemptions and mitigation
- FCC OET Bulletin 65 (Edition 97-01) and Supplement B (amateur radio stations)
Formulas checked against the sources above and our automated test cases on 2026-10-18. Planning aid, not legal or engineering advice.