RF Exposure Calculations for Emergency Commmunication

I’ve calculated some safe distances for RF exposure in typical emergency communication situations. These are for a 5 W HT (handheld radio) or a 50 W mobile, on 2 m and 70 cm, each with typical antennas. The results may also be useful for other VHF/UHF portable activities, like ARRL Field Day, Summits on the Air, or Jamboree on the Air.

Very short version: The 6-7 foot social distance we’ve learned to keep is safe for a typical fixed or mobile em-comm deployment. This is the distance between any part of the antenna, including the radials, and a member of the general population. 5 W HTs are safe for handheld use.

The FCC introduced new RF exposure rules for amateur radio in 2021. Hams used to have special exemptions, now we need to do RF exposure evaluations for all uses. If your transmitter and antenna are like the setup used for these calculations, you might be able to use these results. If yours are significantly different, this should help you get started.

Rf safety sign

The Details

There are different rules for controlled environment (the operator and other people aware of the transmitter) and uncontrolled environment (everyone else). There are also different evaluation methods for handheld transceivers (HTs) and for transmitters (antennas, really) far enough away to give a more even exposure to the whole body.

Handheld Radios

Rf exposure sar

For transmitters very close to a person, the FCC uses specific absorption rate (SAR) evaluations, which are time-consuming and expensive. Those measure localized heating in small areas of the body, either a 1 g or 10 g mass.

This article from the ARRL QEX magazine looks at existing SAR evaluations for commercial radios that use frequencies close to amateur bands.

Amateur Portable Radios (Handheld Transceivers): Exposure Considerations Based on SAR.

The test setup has the HT held 2.5 inches away from your mouth. Based on those measurements, a 5 W HT on 2 m is quite safe. It would need to put out 20+ W to reach the SAR exposure limit. A 5 W 70 cm HT is safe, but probably just under the SAR exposure limit.

Higher-powered HTs, like the 8 W models, are probably over the safe exposure limit on the 70 cm band.

Larger, high-gain antennas might actually be safer. In the ARRL calculator, those would have higher gain and require more distance. In a SAR evaluation, the larger antenna spreads the radiation over a larger area, so it does less localized heating.

This article also uses the occupational exposure limits, similar to the controlled environment limits. And the measurements use a 50% duty cycle, for example, transmitting for one minute, then receiving for one minute. This is a far higher duty cycle than any field operator would have.

The Conclusions section of the QEX article starts with this statement, “A careful but limited examination of SAR test results available in the FCC’s equipment authorization database suggests that handhelds commonly used in the amateur radio service would not exceed exposure regulations based on the magnitude of local SARs.”

Mobile and Temporary Fixed Stations

Rf exposure hf 2

Short version (controlled / uncontrolled), using the worst case across the two bands for each situation.

50 W mobile (field operator): 2.1 feet / 3.6 feet
50 W mobile (net control): 4.7 feet / 6.6 feet

People inside a car with a rooftop antenna are shielded by the car’s metal roof. Exposure is also reduced because the antenna radiates out, not down. The highest emission is straight out, perpendicular to the antenna element. There is little or no emission up or down in line with the antenna. If you can see the antenna through a window, like a trunk-mounted antenna, keep the suggested safe distance.

I used the ARRL RF exposure calculator. I’m including results for a 5 W HT, though the SAR evaluation probably should be used instead.

I used these inputs to the calculation:

Mode duty cycle: 100% (FM).

Transmit duty cycle: for a field operator, one minute out of ten, for net control, five minutes out of ten. Reducing the field operator duty cycle more does not change the results.

HT antenna gain: 2.15 dBi, probably optimistic. This assumes that the antenna and your hand (the other element) act like a 1/2-wave dipole.

Mobile antenna gain: 3.0 dBi (2 m), 5.5 dBi (70 cm). These are the specs for the Comet SBB-5. That antenna is: a) Comet’s most popular mobile antenna, b) the right size to not get caught on trees when roof-mounted, and c) the mobile antenna I own. Other antennas the same length (38 inches) will probably have similar gain, because physics.

Operating frequencies: 146 and 446 MHz.

Include ground reflections? Yes.

Detailed results:

Radio Usage Band (MHz) Environment Distance (feet)
5 W HT field operator 146 controlled 0.55
5 W HT field operator 146 uncontrolled 0.95
5 W HT field operator 446 controlled 0.45
5 W HT field operator 446 uncontrolled 0.78
50 W mobile field operator 146 controlled 1.9
50 W mobile field operator 146 uncontrolled 3.3
50 W mobile field operator 446 controlled 2.1
50 W mobile field operator 446 uncontrolled 3.6
50 W mobile net control 146 controlled 4.3
50 W mobile net control 146 uncontrolled 6.0
50 W mobile net control 446 controlled 4.7
50 W mobile net control 446 uncontrolled 6.6

For more information and a deeper understanding of the issues, dig into the ARRL resources on RF exposure.

If you want safety signs for your deployment, blue “notice” signs posted at the uncontrolled environment limit are a good choice. These might also keep spontaneous volunteers at a respectful distance. Click on this image to go to Flickr, where you can download a full-sized JPG for printing. There is a download icon in the lower right of the image area. That will offer a 2400 x 3000 pixel version.

Rf exposure notice sign

Illustrations are taken from the QEX article and from FCC OET Bulletin 65.


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