getontheairEstimate
Turn on JavaScript to see results. The math is listed below so you can also work it by hand.
How the numbers are worked out
- Average power = PEP × 10(−loss/10) × duty factor × (minutes transmitting ÷ averaging window). The window is 6 minutes for controlled and 30 minutes for uncontrolled.
- EIRP = average power × 10(gain dBi/10). ERP = EIRP ÷ 1.64.
- Distance to meet a limit: R (cm) = √(F × EIRPmW ÷ (4π × S)), where S is the 47 CFR 1.1310 limit in mW/cm² and F is the ground factor (2.56 by default). This is the Supplement B far-field equation.
- Limits (f in MHz): controlled is 100 mW/cm² up to 3 MHz, 900/f² to 30 MHz, 1.0 to 300 MHz, f/300 to 1500 MHz, then 5.0. Uncontrolled is 100 up to 1.34 MHz, 180/f² to 30 MHz, 0.2 to 300 MHz, f/1500 to 1500 MHz, then 1.0.
- Exemption (47 CFR 1.1307(b)(3)(i)(C)): the source is exempt from further evaluation at distance R (m) if the time-averaged ERP is no more than 1,920 R² (0.3–1.34 MHz), 3,450 R²/f² (1.34–30 MHz), 3.83 R² (30–300 MHz), 0.0128 R² f (300–1,500 MHz), or 19.2 R² (1,500–100,000 MHz). R must be at least λ/2π. This tool applies the 30-minute ERP.
Checked against Supplement B Table 4a. For example, 100 W at 14.35 MHz into a 0 dBi antenna at 100% duty gives 0.7 m controlled and 1.5 m uncontrolled, the same as the table. Rule text was checked on eCFR on 2026-10-08.