RF exposure
| RF exposure | |
|---|---|
| What it is | Human exposure to the radio-frequency fields from a transmitting antenna |
| Radiation type | Non-ionizing |
| Main established effect | Heating of body tissue at high levels |
| US rules | FCC §1.1307 and §1.1310, applied to amateurs by §97.13(c) |
| Key ideas | |
| Exposure limit | Maximum permissible exposure (MPE) |
| Strictest limits | 30–300 MHz |
| Main factors | Power, frequency, distance, antenna gain, duty cycle, time |
| Evaluation | Required for every US amateur station since the 2021 rule changes |
RF exposure is the exposure of people to the radio-frequency (RF) energy that a transmitter and antenna produce. RF energy is non-ionizing: unlike X-rays, it does not have enough energy to break chemical bonds, and the established biological effect at high levels is heating of body tissue. The FCC sets limits on how much exposure is allowed, and every US amateur is responsible for making sure their station stays within them, for the operator and household and for neighbors and passersby.
The exposure limits
The FCC's limits are called maximum permissible exposure (MPE). They are expressed as electric and magnetic field strength or as power density, and they vary with frequency. There are two tiers:
- Controlled (occupational) limits apply to people who know about the exposure and can control it, such as the licensee and informed members of the household.
- Uncontrolled (general population) limits apply to everyone else, including neighbors, and are stricter.
The limits are averaged over time, over a shorter period for controlled exposure and a longer one for uncontrolled exposure, so a brief transmission counts for less than a long one. They are strictest between 30 and 300 MHz, the range in which the human body absorbs RF energy most efficiently, and relax at lower and higher frequencies. A given power on the 2-meter band therefore calls for more caution than the same power on 40 meters.
The FCC rules
Under Part 97, a licensee must make sure a station does not expose people to RF above the MPE limits. For many years, amateur stations running less than a set power on each band were excluded from having to evaluate their exposure. Revised rules that took effect in 2021 removed that amateur-specific exclusion. Every station is now covered by the FCC's general exemption criteria, which consider frequency, power and the distance between the antenna and people. A station that falls within those criteria is exempt from a detailed evaluation, but the licensee still has to work out that it qualifies. A station that does not qualify needs a fuller evaluation, and any problem it finds must be fixed. Existing stations were given a transition period to complete their evaluations; new stations and changes to existing ones must be evaluated before they operate.
An evaluation can be done by calculation, computer modeling or measurement. Most hams calculate, using the FCC's OET Bulletin 65 and its Supplement B for amateur stations, or an online calculator such as the ARRL's. The FCC does not ask for the results to be filed, but keeping a written copy is good practice. Every new license applicant certifies that they have read and will comply with the RF exposure rules, and the topic appears on every license exam.
What affects exposure
- Power. Exposure rises with the average power reaching the antenna, which is less than the transmitter's output once feed line loss is counted.
- Distance. Field strength drops quickly with distance. Far from the antenna, doubling the distance cuts power density to about a quarter. Distance is usually the cheapest protection.
- Antenna gain and direction. A Yagi concentrates energy in the direction it points. A beam aimed at a house next door is a different case from one aimed over an empty field.
- Duty cycle. This is the fraction of time a transmitter is at full output. FM and many digital modes, including FT8, transmit at full power for the whole transmission. SSB voice and CW average well below their peak power. Time spent listening also lowers the average, which is why time averaging matters.
Practical steps
- Mount antennas high and away from places where people spend time. An outdoor antenna is easier to keep within limits than one indoors or in an attic.
- Use only the power needed. Halving power costs only 3 dB, about half an S-unit on the other station's signal meter.
- Never touch an antenna while it is transmitting. RF burns are painful and are a separate hazard from field exposure.
- With a handheld radio, keep the antenna away from your face and eyes, and consider a speaker microphone or an external antenna.
- For mobile operating, a roof-mounted antenna puts the car's metal body between the antenna and the occupants.
- Point beams away from people during long, high-power transmissions, and never look into a transmitting microwave dish or feed.