FeedpointHow the distance is worked out
The FCC lets amateurs evaluate exposure with the far-field equations of OET Bulletin 65. Power density at distance R, with the EPA ground reflection factor (field × 1.6, power × 2.56), is
S = 2.56 × EIRP / (4π R²)
Set S to the limit and solve for R. EIRP is the time-averaged power at the antenna times the antenna gain: PEP × mode factor × share of time on the air, less the feedline loss. The result is shown with ground reflection, and without it for comparison.
Controlled and uncontrolled limits
47 CFR §1.1310 sets two tiers. Controlled (occupational) applies to you and to people who know about the exposure and can do something about it, averaged over 6 minutes. Uncontrolled (general population) applies to neighbours, family and passers-by, averaged over 30 minutes. Below 1.34 MHz and above 30 MHz the limits are flat; between, they fall with the square of the frequency, which is why 10 m needs more distance than 40 m at the same power.
| Band | Frequency | Controlled | Uncontrolled | Uncontrolled distance: 100 W SSB, 50 % on the air, dipole |
|---|---|---|---|---|
| 160 m | 2 MHz | 100 | 45 | 3 ⅜″ |
| 80 m | 4 MHz | 56.25 | 11.25 | 6 ¹³⁄₁₆″ |
| 40 m | 7.3 MHz | 16.889 | 3.378 | 1′ 0 ⅜″ |
| 30 m | 10.15 MHz | 8.736 | 1.747 | 1′ 5 ¼″ |
| 20 m | 14.35 MHz | 4.371 | 0.874 | 2′ 0 ⅜″ |
| 17 m | 18.168 MHz | 2.727 | 0.545 | 2′ 6 ¹³⁄₁₆″ |
| 15 m | 21.45 MHz | 1.956 | 0.391 | 3′ 0 ⅜″ |
| 12 m | 24.99 MHz | 1.441 | 0.288 | 3′ 6 ⅜″ |
| 10 m | 29.7 MHz | 1.02 | 0.204 | 4′ 2 ⅜″ |
| 6 m | 54 MHz | 1 | 0.2 | 4′ 2 ⅞″ |
| 2 m | 148 MHz | 1 | 0.2 | 4′ 2 ⅞″ |
| 1.25 m | 225 MHz | 1 | 0.2 | 4′ 2 ⅞″ |
| 70 cm | 450 MHz | 1.5 | 0.3 | 3′ 5 ⁹⁄₁₆″ |
| 33 cm | 928 MHz | 3.093 | 0.619 | 2′ 4 ¹⁵⁄₁₆″ |
| 23 cm | 1,300 MHz | 4.333 | 0.867 | 2′ 0 ⁷⁄₁₆″ |
Mode factors and time on the air
Average power is far below PEP for most voice modes. OET-65 Supplement B, Table 2, gives the factors used here. Modes it does not list are taken as constant envelope, the safe side, and marked as an estimate.
| Mode | Factor | Basis |
|---|---|---|
| SSB voice | 0.2 | OET-65 Supplement B, Table 2 |
| SSB voice, heavy processing | 0.5 | OET-65 Supplement B, Table 2 |
| CW | 0.4 | OET-65 Supplement B, Table 2 |
| FM voice | 1.0 | OET-65 Supplement B, Table 2 |
| RTTY | 1.0 | OET-65 Supplement B, Table 2 |
| FT8, FT4 and other digital | 1.0 | OET-65 Supplement B, Table 2 |
| Carrier (tuning) | 1.0 | OET-65 Supplement B, Table 2 |
| Packet | 1.0 | Not listed: constant envelope assumed (estimate) |
| SSTV | 1.0 | Not listed: constant envelope assumed (estimate) |
| AM voice | 1.0 | Not listed: constant envelope assumed (estimate) |
| ATV, AM | 1.0 | Not listed: constant envelope assumed (estimate) |
| ATV, vestigial sideband | 1.0 | Not listed: constant envelope assumed (estimate) |
Time on the air is the busiest part of the averaging window. With an on/off cycle the calculator takes the window that starts with a key-down: 5 minutes on and 5 off fills a whole 6-minute window (100 %), but only 15 of 30 minutes.
The exemption test
The FCC rules of 2020 added §1.1307(b)(3)(i)(C): it exempts a station whose ERP at a given distance is at or below a threshold, for example 3450 R² / f² watts between 1.34 and 30 MHz. Enter the distance to people and the calculator runs the test with your 30-minute average ERP. The table is only valid at least λ/2π from the antenna; closer than that, the full evaluation above applies.
Close to the antenna
Inside λ/2π the far-field equations do not strictly hold. OET-65 notes that for wire antennas they over-predict there, which errs on the safe side, so the result still stands with a note. Small transmitting loops are the exception: their near field is stronger than the estimate, so tick Magnetic loop and the calculator gives no distance inside λ/2π.
Outside the United States
This page uses the US rules. The app also evaluates by the German BNetzA method with the 26. BImSchV limits, by the ICNIRP 2020 general public levels that Ofcom requires in the UK, and by ICNIRP 2020 or 1998 for other countries, and saves each evaluation as a dated record.
Sources
- 47 CFR §1.1310(e)(1) Table 1, Limits for Maximum Permissible Exposure
- 47 CFR §1.1307(b)(3)(i)(C) Table 1, single RF source exemption thresholds
- FCC OET Bulletin 65, Edition 97-01, eq. (4) and (7)
- FCC OET Bulletin 65 Supplement B (amateur), Table 2 and the averaging examples
Calculations are aids; go by the rules in force and your own measurement.
Checked against the sources on November 2, 2026.