Every amateur station must keep people out of RF fields stronger than the FCC allows, and the licensee is responsible for checking (47 CFR 97.13(c)). The FCC notes that most amateur transmitters would not normally expose people above the limits, because of their modest power, intermittent transmissions and antennas that are hard to reach (FCC RF Safety FAQ). You still have to check. This guide explains the rules, the limits and the methods.
What RF does to the body
Radio waves are non-ionizing radiation. Unlike X-rays and gamma rays, they do not carry enough energy to strip electrons from atoms and damage DNA. The established hazard is heating: very high RF levels can heat body tissue faster than the body can carry the heat away. The eyes and testes are particularly vulnerable because they have relatively little blood flow to remove heat (FCC RF Safety FAQ).
How much energy the body absorbs depends on frequency. A standing adult absorbs the most around 70 MHz, so exposure limits are strictest from about 30 to 300 MHz (FCC RF Safety FAQ). Touching an antenna while it transmits can also cause an RF burn (Technician pool, T0C07).
The rules that apply to amateurs
Three sections of the rules work together:
- 47 CFR 97.13(c) applies the exposure rules to amateur stations and names OET Bulletin 65, Supplement B, as the guidance for evaluating amateur stations.
- 47 CFR 1.1307(b) says when a station is exempt from a routine evaluation and what to do if it is not.
- 47 CFR 1.1310 sets the limits.
The limits come in two tiers. Occupational/controlled limits apply to people who know about the exposure and can control it. General population/uncontrolled limits apply to everyone else and are lower (47 CFR 1.1310). Part 97 lets you evaluate exposure of your own household against the controlled limits, provided you and they have had appropriate training and information. Neighbors and everyone else must be evaluated against the uncontrolled limits (47 CFR 97.13(c)(1)).
What changed in 2021
Before 2021, Part 97 exempted amateur stations from routine evaluation based on transmitter power alone. In its 2019 order (FCC 19-126), the FCC replaced that power-only exemption with the general exemption criteria that apply to all rule parts (85 FR 18131). The new 1.1307 rules took effect on May 3, 2021 (86 FR 20456). Previously exempt sites got two years from the effective date to complete their evaluations (85 FR 18131), a transition that the ARRL notes ended on May 3, 2023 (ARRL). The exposure limits themselves did not change; the FCC concluded the evidence supported keeping them (FCC).
Maximum permissible exposure
Between 300 kHz and 6 GHz, compliance can be judged against maximum permissible exposure (MPE) limits instead of measuring absorption in the body, and above 6 GHz MPE is always used. The power density limits from Table 1 of 47 CFR 1.1310, in milliwatts per square centimeter, with f in MHz:
| Frequency (MHz) | Controlled | Uncontrolled |
|---|---|---|
| 0.3 to 1.34 | 100 | 100 |
| 1.34 to 3.0 | 100 | 180 / f² |
| 3.0 to 30 | 900 / f² | 180 / f² |
| 30 to 300 | 1.0 | 0.2 |
| 300 to 1,500 | f / 300 | f / 1,500 |
| 1,500 to 100,000 | 5 | 1.0 |
The limits are averaged over time: 6 minutes for controlled exposure and 30 minutes for uncontrolled. Higher levels are allowed briefly as long as the average stays within the limit (47 CFR 1.1310(d)(4)). Some examples for the uncontrolled tier, computed from the table: 180 / 14.2² = 0.89 mW/cm² on 20 meters, 0.2 mW/cm² on 6 and 2 meters, and 446 / 1,500 = 0.30 mW/cm² on 70 cm. The 30 to 300 MHz range has the lowest limit, matching the body’s absorption peak described in the FCC RF Safety FAQ.
What changes your exposure
- Power at the antenna: transmitter output minus feed line loss (ARRL).
- Duty cycle: the percentage of time the transmitter is transmitting during the averaging period (Technician pool, T0C11). The ARRL calculator also applies a mode duty cycle: 20 percent for conversational SSB without speech processing, 50 percent with heavy processing, 40 percent for conversational CW, and 100 percent for FM, AM, RTTY, AFSK, a steady carrier and any unknown mode. It then accounts for how many minutes you transmit and receive (ARRL calculator). Halving the duty cycle doubles the allowable power density (Technician pool, T0C03).
- Distance: power density falls off rapidly as you move away from an antenna (FCC RF Safety FAQ).
- Frequency: the limits change with frequency, as the table shows.
- Antenna gain and pattern: the radiation pattern affects exposure (Technician pool, T0C04), and the ARRL says antennas with even moderate directionality may need to be evaluated in multiple directions. For low or non-directional antennas, the ARRL recommends including ground reflections in the calculation as a more conservative estimate (ARRL).
How to evaluate your station
Step 1: check for an exemption. A source with a time-averaged power of 1 mW or less is exempt. Otherwise, use Table 1 of 47 CFR 1.1307(b)(3). With R the closest distance in meters between any part of the antenna and any part of a person, and f in MHz, the station is exempt if its ERP in watts is no more than:
| Frequency (MHz) | Threshold ERP (watts) |
|---|---|
| 0.3 to 1.34 | 1,920 R² |
| 1.34 to 30 | 3,450 R² / f² |
| 30 to 300 | 3.83 R² |
| 300 to 1,500 | 0.0128 R² f |
| 1,500 to 100,000 | 19.2 R² |
Here ERP is the time-averaged power delivered to the antenna times its maximum gain relative to a dipole, and the table applies only when R is at least one wavelength divided by 2π (47 CFR 1.1307(b)).
- HF example. 100 W on 14.2 MHz into a dipole (gain 0 dBd), assuming continuous transmission. The wavelength is 300 / 14.2 = 21.1 m, so R must be at least 21.1 / 2π = 3.36 m. At that distance the threshold is 3,450 × 3.36² / 14.2², about 193 W. Since 100 W is less, the station is exempt if no one can get within about 3.4 m (11 feet) of the antenna.
- VHF example. 50 W FM on 146.52 MHz into an antenna with 0 dBd gain, transmitting continuously. Solving 50 = 3.83 R² gives R = 3.6 m (about 12 feet), which is more than the minimum of λ/2π = 0.33 m. If anyone can be closer than 3.6 m, for example passengers in a car with the antenna on its roof, the station is not exempt and needs an evaluation. That does not mean it fails: ERP is based on the time-averaged power delivered to the antenna, so time averaging and feed line loss both count (47 CFR 1.1307(b)).
Step 2: if not exempt, evaluate. Acceptable methods include calculation based on OET Bulletin 65, computer modeling, and measurement of field strength with calibrated equipment (Technician pool, T0C06), and Part 97 names OET Bulletin 65, Supplement B, as the guidance for amateur stations (47 CFR 97.13(c)). The FCC notes that Bulletin 65 “is currently under review to provide updated guidance regarding the rule changes of FCC 19-126” (FCC OET), so use it alongside the current rule text. The ARRL RF Exposure Calculator takes your power, duty cycle, transmit time, antenna gain in dBi and frequency, and returns minimum compliance distances for controlled and uncontrolled environments. It is not meant for antennas closer than 20 cm (8 inches) to a person, and the ARRL suggests running it several times to account for the antenna’s lobes and directionality. Run it for each band, mode and antenna.
Step 3: fix any problem and keep records. If people could be exposed above the limits in an accessible area, you must prevent it (47 CFR 97.13(c)(2)). The FCC can ask a licensee for the technical basis of an exemption or evaluation (47 CFR 1.1307(b)(1)), so keep your numbers with your station records. Re-evaluate the station whenever you change anything in the transmitter or antenna system (Technician pool, T0C09).
Practical steps
- Relocating antennas reduces exposure (Technician pool, T0C08). Mount antennas high and away from where people sit, walk and sleep; the FCC credits the relative inaccessibility of most amateur antennas as one reason most stations stay within the limits (FCC RF Safety FAQ). If you use an indoor antenna, make sure occupied rooms stay within the limits.
- If a beam could exceed the limit toward a neighbor’s yard, do not point it there when people are present.
- Use the minimum transmitter power necessary, which the rules require anyway (47 CFR 97.313(a)), and favor lower duty cycle modes for long transmissions at high power.
- Keep people, including you, from touching antennas during transmission.
- Explain RF safety to your household if you rely on the controlled limits for them.
The electrical safety guide covers shock, lightning and tower hazards.