RF exposure evaluation: what the FCC requires and how to do it

For decades most amateurs never thought about RF exposure, because the rules contained a categorical exemption that excused amateur stations below a stated power on each band. That exemption is gone. The FCC removed the amateur-specific carve-out and replaced it with the same general exemption criteria the rest of the radio services use, and the transition period ended in 2023. There is now no power level at which an amateur station is simply assumed to comply.

This sounds heavier than it is. For a typical station the evaluation takes about twenty minutes, most stations pass comfortably, and the ones that fail almost always fail for one obvious reason: an antenna hung close to where people actually stand. What the rule really does is force you to notice that reason.

Nothing on this page evaluates your station, because nobody can. The answer depends on your frequency, your power at the feedpoint, your antenna gain in the direction of the person, your duty cycle and how close people get. Run the RF exposure calculator with your own numbers and keep what it tells you.

What the rule actually says

The governing text is 47 CFR Part 1 Subpart I and Part 97.13(c), published at the electronic Code of Federal Regulations. Two ideas carry the whole thing.

The first is maximum permissible exposure, or MPE. This is a limit on how much radio frequency energy a person may absorb, expressed as power density in milliwatts per square centimetre, and it varies with frequency because the human body absorbs energy differently at different wavelengths. The body is most efficient at absorbing energy where it is roughly resonant, which lands between about 30 and 300 MHz. That is why the limit is at its strictest across 2 metres and the low VHF bands, and why a 50 W mobile rig on 2 metres deserves more thought than a 100 W station on 80 metres.

The second is the split between a controlled and an uncontrolled environment. A controlled environment is one where the people present know they are being exposed and can act on it: you, in your own shack, aware of the antenna and able to stop transmitting. An uncontrolled environment is everybody else, which in practice means your family, your neighbours, and anyone on the pavement outside. The uncontrolled limit is five times stricter than the controlled limit, and it averages over 30 minutes rather than 6.

The four numbers you need before you start

An evaluation is arithmetic on four inputs. Getting the inputs honest matters far more than the arithmetic.

Power at the antenna, not power out of the radio

Start with your transmitter output, then subtract the feedline loss between the radio and the antenna. A 100 W station feeding 100 ft of RG-58 on 2 metres delivers well under half that to the antenna. Work it out on the coax loss calculator rather than guessing, because on VHF the difference is large enough to change the answer.

Antenna gain in the direction that matters

Use the gain toward the person, not the gain in the main lobe. A beam pointed at the horizon has far less gain straight down at the family in the garden, but a vertical has most of its gain in exactly the plane where people stand. Where you cannot find a real pattern, using the published main-lobe gain is the conservative choice and costs you nothing if you pass anyway.

Distance to the nearest person, in metres

The nearest place a person can actually be, not where they usually are. That is the neighbour bedroom on the other side of the fence, the pavement under the wire, and the ladder you stand on to clean the gutters. Distance is the strongest lever in the whole calculation, because power density falls with the square of it.

Duty cycle, honestly

Averaged over the exposure window, not peak. FT8 transmits for roughly 13 seconds out of every 15 second slot and is close to 100 percent duty while transmitting, so a run of FT8 is far harsher than the same power on SSB, where an average voice conversation lands somewhere around 20 percent once you account for both the syllabic gaps and the half of the time you are listening.

Running the evaluation

What a failing evaluation looks like, and what to do

Almost every failure this hobby produces is one of four situations, and none of them are subtle once you have measured the distances.

• A VHF or UHF antenna within a couple of metres of a person. This is the classic. A 50 W mobile rig with a roof-mounted antenna and a passenger head a metre away, or a 2 m vertical on a balcony rail beside a chair. The band range where the MPE limit is strictest is exactly the band range most beginners start on. Raise the antenna or move the person.
• A wire antenna at head height across the garden. An end fed half wave sloping from the eaves to a fence post passes through the space where the washing line is. The high-voltage end of an end fed antenna is the worst place to be, and it also happens to be the end people can reach. Put the far end above head height and out of reach, and put the feedpoint end where nobody walks.
• An attic antenna over an occupied room. The floor of an attic is not RF shielding. A dipole in the loft above a bedroom puts the antenna two metres from a sleeping person for eight hours a night, which is the one scenario where the 30 minute averaging window offers no help at all.
• High duty digital at full power on a small lot. A station that passes easily on SSB can fail on FT8 at the same power, because the duty factor more than quadruples. If you run digital modes, evaluate at digital duty and either turn the power down for those modes or gain the distance.

Gear that makes the evaluation easier to satisfy

Nothing here is required. These are the items that turn a failing evaluation into a passing one, or that let you measure rather than assume.

What actually helps

Height, real measurements and a way to know your true power. Prices researched at time of writing.

Easy Up

Easy Up 20 ft telescoping push-up mast

The cheapest metre of distance you can buy. Getting a wire or a vertical up and away from the patio is usually the entire fix.

Check price$185.00

AURSINC

NanoVNA H4 vector network analyzer

Tells you what your antenna is actually doing on each band, which is the input the whole calculation depends on. Guessing the resonant point guesses the gain.

Check price$89.90

Surecom

Surecom SW-102 digital VHF/UHF SWR meter

An inline power and SWR meter reads your real forward power at the feedpoint end, rather than the number on the front panel of the radio.

Check price$57.99

Bolton Technical

Bolton400 low-loss coax, 100 ft with PL-259

Low-loss coax lets you site the antenna far from the house without throwing away the power that made the distance worthwhile.

Check price$119.95

Common misreadings of the rule

"I run QRP so it does not apply to me"

The evaluation requirement applies at every power level. What changed in 2021 is that low power no longer exempts you from evaluating. In practice a 5 W station passes easily at almost any sane distance, so the evaluation is quick, but you still do it and still keep it.

"Someone will send me a certificate"

There is no certificate, no filing and no fee. You perform the evaluation, you record the result, and you keep it. Nobody issues an approval.

"My antenna is a dummy load so I am fine"

A dummy load still radiates a little and a poorly built one radiates more than you expect, but the real issue is that people who reason this way tend to apply it to attic antennas and short verticals as well. Evaluate the antenna you transmit into.

"The rule is about long-term health effects"

The MPE limits are set to prevent tissue heating, which is a measured, immediate, well-characterised effect. The averaging windows exist precisely because heating is a thermal process with a time constant. Whatever your view of long-term effects, the limits are not built on them.

"Receiving counts"

It does not. Exposure evaluation concerns your transmitted power. You are free to listen with anything, anywhere, at any age, without a licence and without an evaluation.

Keeping the record

Keep the evaluation with your station documentation and revisit it whenever anything in the four inputs changes: a new antenna, a new band, more power, an amplifier, a move, or a neighbour building an extension that puts a bedroom where the shed used to be. A dated sheet per antenna, with the calculator output attached, is a complete record. Store it where you can find it, because the only time it matters is the one time somebody asks.

If you are building a station from scratch, the evaluation is easier to satisfy when you plan for it. Both the first HF station build and the full HF base station build place the antenna deliberately rather than wherever the wire reaches, and both list the mast and feedline that make the distance possible.