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.

Windshield Survey: A COVID-Friendly Emergency Service Project (E. Prep. 7a)

Emergency Preparedness Merit Badge requirement 7a is “Take part in an emergency service project, either a real one or a practice drill, with a Scouting unit or a community agency.” How do you do this while Scouting at home?

A standard part of our city emergency drills could be adapted as an emergency service project. In a disaster, our emergency volunteers quickly collect information about damage with a “windshield survey” or “windshield damage assessment”. That information is collected centrally.

Volunteers make notes of the damage in their neighborhood and report it to our volunteer operations center. The damage could be to houses, water mains, gas lines, roads, or power lines. Information about injuries is also collected. In an actual disaster, this would be forwarded to our city EOC for city-wide situational awareness and to dispatch our professional or volunteer emergency response teams.

As I write this, our next drill is tomorrow evening. I’ll be at our volunteer operations center running a two-way radio net to collect this information from neighborhood volunteers.

Our city Damage Assessment Form collects summary information on the front and has instructions on the back.

Minor damage

*Minor, repairable damage.*

Emergency Service Project

Organize a “windshield survey” or “windshield damage assessment”. This is done by walking or driving an area and making notes of the damage. For our drills, it is earthquake damage, but it could also be from a windstorm or other disaster.

Each drill has a list of fake incidents, so “water main broken at Ferne and Leaf”, “gas leak at 1120 Ferne”, and so on. The lists are distributed to the volunteers in the matching neighborhood. Each local team enters the their incidents on a damage assessment form, then reports the incident summary to the central collection point, “net control”. Net control enters the data on their own copy of the form. At the end of he drill, we check that all the incidents were reported and transmitted properly.

Another approach would be to list things are already in the neighborhood, like “blooming flowers in front”, “porch light on”, “two cars in driveway”, “boat”, and so on. For the drill to work well, those should be present in the neighborhood but not at every address, maybe one each per block. Those reports get rolled up for a block or neighborhood, then called in to net control.

Reports could be sent through phone calls, text messages, emails, or FRS two-way radios. Our volunteers use an internet app if available, but also practice with radios that work without the Internet. The Scout organizing the drill gets to choose thee communication technology.

The Scout should do a dry run, with a couple of checks on nearby streets to see if incident collection works, then report to a helper using the damage assessment form. After that, make the final damage assessment forms, make fake damage data if needed, organize the participants, including teaching them how to use the form, then run the drill.

The entire drill can be run without making in-person contact. Training can be remote. Reporting is not face to face.

Your city emergency response volunteers may already do this. The fire department almost certainly knows how to do this kind of assessment.

Update: From a Facebook comment: You could drive home the seriousness of the service by having each of those examples STAND FOR a serious issue that would have the same rate of incidence – give a translation sheet that says, for example, flowers blooming out front gets marked as “small tree limbs down”, a house with two cars gets marked “vehicle damage”, a home with a flag gets marked as “broken windows” and a home with a full size flag on a flagpole gets marked as “hazardous structural damage”.

BaoFeng HTs and Spurious Emissons

The January edition of QST has some disturbing data about dirty transmitters in BaoFeng HTs.

Amateurs are responsible for their transmitters being clean, but most of us don’t have the test equipment to check that. Also, manufacturers must meet the FCC regulations for every transmitter sold.

The ARRL Lab set up at hamfests and tested the HTs that hams had with them. Over four years, only 5% to 9% of BaoFeng HTs passed the test. Alinco, Icom, Kenwood, and Yaesu had 100% pass rates. Wouxon improved from 83% to 100% over the years.

QST 2020 01 HT Testing

From “Technical Correspondence”, QST, January 2020, pages 60-62. This chart is on page 61. QST is available online to ARRL members. This is a link to the article online.

The FCC rules for spurious emissions are in 47 CFR § 97.307 – Emission standards.

What if you already own a BaoFeng, like me?

Run low power. This will reduce the amount of power in the spurious emissions. Reducing the power from 5 W to 0.5 W should reduce the spurious emissions by 10 dB. The spurs still won’t be 40 dB below the carrier, but they will be lower in terms of absolute power. It can’t hurt. It will make your battery last longer, too.

What if you want an inexpensive HT?

Instead of a $50 BaoFeng, save up a bit more for a $75 Yaesu FT-4XR. From my research, this is the only HT under $100 from a major radioo vendor.

The FT-4XR uses the same chipset as the BaoFeng, so it has roughly the same feature set. But it has a clean transmitter and better interference rejection in the receiver. It also has ham-specific firmware, like automatic repeater offsets. That should make it easier to use.

The FT-4XR also uses the same antenna connector as BaoFeng, so aftermarket Nagoya antennas might fit. I would probably try the Nagoya NA-771 for $17, which might fit. I’ve heard recommendations for the Diamond SRJ77CA ($27).

I linked to DX Engineering’s page above, but the FT-4XR is available at similar prices from Ham Radio Outlet, Gigaparts, and other ham stores. Amazon has it at a higher price ($83), oddly.

BaoFeng UV 5R Yaesu FT 4XE

How did this happen?

It appears that the BaoFeng radios were designed for the much more lenient Part 90 emission regulations and do not meet the amateur radio regulations.

An article by AD5GG compares BaoFeng UV-5R emissions to Part 90 (private land mobile) and Part 97 (amateur) regulations. The BaoFeng meets the weaker Part 90 limits, where spurious emissions are not to exceed -20 dBc (dBc is relative to carrier). Part 97 limits spurious emissions to -40 dBc, 100X lower than the Part 90 limit.

Designing for the Part 97 limits requires additional low-pass filtering on the output. The new parts may only be pennies, but it would require a new board design. Maybe new versions of the BaoFeng HTs will be designed to the stricter standards, but I’ll have to see proof of that. The UV-5R is up to the third generation, at least, and still not compliant.

Plastic Pipe Roof Antenna Support

I noticed a clever antenna mount on another ham’s roof, so I built one myself. Putting my VHF/UHF antenna at the highest point of the roof has really improved my ability to copy some of the far-flung participants in our weekly ARES/RACES net.

plastic pipe antenna mount close-up

A cradle built from two-inch ABS DWV (drain, waste, and vent) pipe sits across the ridge of the roof. Legs two feet long go down on each side and a two-foot section is a vertical antenna mast.

My antenna is a Diamond X50NA, same as the X50A, but with a weatherproof Type N connector. I did additional weatherproofing with 3M Temflex 2155 rubber splicing tape and Scotch Super 33+ electrical tape.

I had spotted an antenna mounted like this and contacted the ham at that address. Rolf Klibo, N6NFI, replied with an article he’d written for the SPARK newsletter describing the mount. With that, I was off to the hardware store.

I used two-inch ABS DWV (drain, waste, and vent) pipe in two-foot sections. I cut up two of them to make the four pipe sections that go along the ridge of the roof.

The X50NA mounting hardware fits masts up to 2 3/8 inch, which is why I used two inch pipe. For coax strain relief, I used a conduit hanger that fit. Zip ties attach a loop of coax to the hanger so the full weight of the feed line isn’t pulling on the connector. That can rip the coax right out of the connector, or harder to diagnose, pull just one of the shield or center connector loose.

Antenna mount 3 Antenna mount 4

For a commercial version, I’d look at the Rohn NPPK, a steel frame that fits over the roof peak. It is designed to have a rubber mat underneath and four 18 pound concrete blocks holding it down. I’m sure it is far more secure than my DIY plastic pipe mount.

Using a Mobile Antenna as a Temporary Base Antenna

For our July Fourth Safety Watch this year, I used my dual-band mobile antenna on a ground plane mount on a camera tripod. I’d purchased a Nagoya GPK-01 NMO Ground Plane Kit ($28) to test my NMO mobile antenna, because my mag mount seemed flaky.

As I was drifting off to sleep one night, I thought that the 1/4-20 screw on my camera tripod might fit the holes on the ground plane kit. It did, so now I have a robust, free-standing, dual-band antenna for em-comm use.

IMG 4753

The antenna is a Comet SBB-5NMO with 3 dB of gain on 2 m. Not a flamethrower, but a solid antenna when used with a good ground plane.

The radials unscrew from the mount, so the ground plane kit packs up small.

IMG 4750

Our station was up on a hill, so we had great line of sight to the whole valley. We ran a mobile rig (Yaesu FT-8900R) at 50 W from a battery, so we had a great signal. We could also hear the other stations clearly, so our station was net control.

IMG 4748

The only thing I’ll change next time is to use 1/4-20 wingnuts instead of regular nuts. I might paint them orange, too. I was sure that I would drop a nut in the grass and never find it.

And yes, that is a classic Gitzo Reporter aluminum tripod. If needed, I could extend the center column to get the radials above eye level.