RF Exposure Calculator for Amateur Stations

This is the one calculation on this site that is not optional. The FCC requires every amateur station to evaluate its own RF exposure and to be able to demonstrate compliance, and the categorical exemption that used to excuse amateurs by power level was removed. There is no wattage below which you may simply assume you are fine. The arithmetic takes ten minutes and the answer is usually reassuring, which is exactly why it is worth doing rather than avoiding.

What the evaluation actually asks

The question is not whether your station is dangerous. It is whether the power density at the places people actually are, averaged over the appropriate window, exceeds the maximum permissible exposure for the frequency you transmit on. That breaks into five numbers, and four of them you already know.

1. Frequency. Determines the limit, and the limit varies by more than a factor of five across the amateur spectrum.
2. Power at the antenna. Transmitter output minus feedline loss. On a long UHF run this can be less than half the transmitter figure, which works in your favour here even though it works against you everywhere else.
3. Antenna gain. In dBi, and in the direction that matters. Gain concentrates power, so a beam pointed at a neighbour is a completely different case from the same beam pointed elsewhere.
4. Duty cycle of the mode. How much of the time during a transmission the carrier is actually at full power.
5. Transmit fraction. How much of the averaging window you spend transmitting at all.

Multiply power by duty cycle by transmit fraction to get the average power, apply antenna gain, and compare the resulting power density at the distance of interest against the limit. That is the whole evaluation.

The limits, band by band

Two features of that table drive everything. Below 30 MHz the limit falls as the square of frequency, so 10 metres is far more restrictive than 160 metres. Between 30 and 300 MHz the limit sits at its floor, 0.2 mW per square centimetre uncontrolled, which is the strictest anywhere in the spectrum. That is 6 metres, 2 metres and 1.25 metres, and it is precisely where handheld radios put an antenna next to a human head.

Duty cycle is the number people get wrong

The consequence is worth stating plainly: a 30 watt FT8 station is a worse exposure case than a 100 watt single sideband station. Single sideband with no processing averages a fifth of peak power, so 100 watts becomes 20. FT8 transmits full carrier for the whole 12.6 second transmit period, so 30 watts stays 30. Digital operators running unattended at low power routinely assume they are the safe case and are frequently not.

The transmit fraction multiplies this again. Averaging is over 6 minutes for a controlled environment and 30 minutes for an uncontrolled one, so a station transmitting one minute in five is averaged accordingly.

Three worked cases

A 5 watt handheld held to the face

Two metres at 146 MHz, 5 watts, a stock rubber duck at roughly 0 dBi, FM at 100 percent duty, transmitting perhaps 20 percent of a 30 minute window. Average power is 1 watt. The antenna is two inches from your head. This is the case with the least margin in normal amateur operation, and it is why the standard advice is not to hold a transmitting handheld against your temple.

The cheap and complete fix is to move the antenna away from your skull. A Baofeng BF-850 speaker mic$12.99 puts the radio on your chest for about thirteen dollars, and a Tactical chest rig radio pouch$17.99 does the same with the antenna clear of your head. Both are better answers than reducing power, because both preserve the contact.

A 50 watt mobile with a roof-mounted antenna

Two metres, 50 watts, an NMO whip at about 3 dBi on the roof, FM at 100 percent duty, half the window transmitting. Average power 25 watts, and the antenna is roughly three feet above the occupants with the vehicle roof acting as a ground plane between. The roof is doing a great deal of work here, and the case that matters is not the driver but a pedestrian standing beside the vehicle while the operator holds a long transmission.

A 100 watt HF station with a wire near a property line

Forty metres at 7.15 MHz, 100 watts, an end fed half wave at about 0 dBi, single sideband at 20 percent duty, half the window transmitting. Average power 10 watts. The limit at 7 MHz is 3.5 mW per square centimetre uncontrolled, which is far more permissive than 2 metres, so the compliance distance is small. What still needs checking is the far end of the wire, where the voltage maximum sits, and whether that end is over a neighbour's garden. Covered on the end fed half wave calculator.

What to do if the number is uncomfortable

• Move the antenna. Distance is the strongest lever available because power density falls as the square of it. Doubling the distance quarters the exposure.
• Raise it. Height is distance, and it also moves the main lobe away from people rather than through them.
• Point it somewhere else. Gain is directional. A beam turned away from the property line is a different evaluation from the same beam turned towards it.
• Reduce power. Effective, and the least popular option. Halving power reduces the compliance distance by about 30 percent, which is less than people expect because of the square law.
• Change duty cycle or operating pattern. Genuinely useful for digital modes, where dropping from continuous unattended operation to intermittent operation changes the average substantially.

Keeping the record

There is no prescribed form and no requirement to file anything with anyone. What is required is that the evaluation was performed and that you can show your working. Print the numbers you used, note where people actually are relative to the antenna, sign and date it, and put it in the station folder with your license copy.

Redo it whenever anything material changes: a new antenna, a change of height, an amplifier, a new mode, or a neighbour building an extension closer to your antenna than the old fence line. An amplifier is the change most likely to move a compliant station out of compliance, since power density scales linearly with power. The full guide with worked worksheets is on the RF exposure evaluation guide.