Sunday, April 25, 2021

Licensed Hams - A New Rule on May 3rd

The American Radio Relay League reminds all amateurs that in case you've missed it, the new RF exposure rules take effect on Monday, May 3rd

The FCC has announced that rule changes detailed in a lengthy 2019 Report and Order governing RF exposure standards go into effect on May 3, 2021. The new rules do not change existing RF exposure (RFE) limits but do require that stations in all services, including amateur radio, be evaluated against existing limits, unless they are exempted. For stations already in place, that evaluation must be completed by May 3, 2023. After May 3 of this year, any new station, or any existing station modified in a way that's likely to change its RFE profile - such as different antenna or placement or greater power - will need to conduct an evaluation by the date of activation or change.

I did a fairly deep dive into the requirements last May, before I had my Ham Radio Series, so they're not on that separate page, but the first part is "A Little Radio Safety Techno-Geekery", and some examples of how to go about the calculations are in the second part, "A Little More on RF Power Safety and Ham Radio."  Besides my writings, the league itself offers things to help you.

"RF Exposure and You" is available in PDF format for free download from ARRL at,
ARRL also has an RF Safety page on its website at,
The ARRL also has a worksheet that helps in the process.

Here's the odd part.  There are no instructions that this is to be filed with the FCC or anyone else.  I suppose that means you're expected to do the calculations, and keep them on file in case you're questioned at some point.  I would presume that would be if a neighbor complains you're interfering with their TV or Stereo or other form of entertainment and an investigation is started.  Thankfully, with the switch to HDTV and cable, those complaints seem to be fewer than back with analog TV.  

As the ARRL points out, the major change in the rules is that before now, hams were generally exempt from this requirement.  That exemption goes away.  

There is no preferred format stated for the analysis, but sticking with that ARRL worksheet might be a good thing. 

As a quick review, let's look at table 1 in the first of my two posts.  This shows the power at the antenna that requires analysis:

If you operate a typical, off the shelf ham transmitter/transceiver with 100 Watts output power, you don't have to address any exposure until you get to the 12m band, because there are unavoidable losses between the output of the transmitter and the antenna, and even tenths of a dB loss will get you below the 100 Watts that requires evaluation on 15m.  

The next table in that article tells you the distances (in meters) from an antenna with one of four gains where the power is expected to exceed the FCC power density limits.  Always use the "Unc." column, for uncontrolled exposure - note those are always farther away, which means lower power.  The "Con" column, controlled exposure, is for people working in the industry, expected to be more aware of the need for caution.

Note that you may reduce power by the transmit duty cycle because these are time averaged powers; the chart says 50% and without lots of effort to measure that, it isn't unrealistic.  Even "full power, full time" modes like RTTY (Radio Teletype) don't turn on the transmitter and leave it on for minutes on end.  The league says to use 40% of power for CW, 20% for SSB with no speech processing, 40% for SSB with heavy speech processing,  You should also reduce the power getting to the antenna by the loss in the transmission line to the antenna. 

If I reduce the 100W power to 85% to account for the transmission line (following the example in the second article) that leaves 85 watts at the antenna.  If I reduce that to 30% for the transmitter duty cycle, it's now at 25.5 Watts, far short of the 75 W that requires analysis by the first table.  Notice that each frequency band calculates the powers for four different antenna gains.  In the very simple cases, say your 12m antenna has 3 dB gain.  That means the distance from the antenna where the power exceeds the safety limit is 3.8 meters, or 12-1/2 feet, in the direction the antenna is pointed.  If nobody is closer than 3.8 meters from the antenna, you're good. 

That's even below the power to the antenna that requires analysis for the 10m band.  

I think that between the various resources I've linked to here, both the ARRL's handouts and my articles, you should have what you need to determine how to do the analysis for your station.  As a general approach, there's nothing wrong with making worst case assumptions.  Saying you're delivering the full 100 Watts to the antenna - or a full 100% duty cycle.  That way you know you're always delivering less power to anyone near the antenna and that they're always safer than the limits. 


  1. What's the point of such a rule....which will of course have the force of law even though it was never voted on by an elected politician? It's to provide a .Gov agency ONE MORE TOOL to persecute someone if that someone runs afoul of Big Brother. The Federal Register has literally THOUSANDS of such "regulations", all of which can be used to utterly destroy a citizen if some bastard working for the Fed Gov decides said citizen needs destroying. This is just one more.

  2. Because relatively low power HAM stations need EM exposure safety rules, but megawatts from space is a nifty-keen idea.

    1. Yeah, but if you radiate 10 million watts from 200 miles up, it's less than a billionth of a watt at the surface.

    2. Oops. Left out an important part; that calculation is for 10.0 GHz.

    3. I simply can't take seriously any of the proposals of electric power beamed directly from space. It either doesn't provide nearly enough power density, or way too much.

    4. Oh, sure. I used to call it the "Microwave engineers full employment act."

      The problem (same as with all energy sources) is that physics is a bitch. You want a low enough power density so that the radio coming down doesn't fry birds or the electronics in airplanes, and that means paving over entire states with receiving antennas. Unless you want to be like the solar power people that actually do incinerate birds.

      Remember solar energy input is about 1300 watts per square yard, but you can only extract about 1/4 of that with photovoltaic cells.

      If you want less area on the ground, you have to increase the power density which means bigger chances of frying birds of both the flesh and blood and the man-made kinds.

      In the race for power sources, power density wins.

  3. I guess you couldn't pretzel the 2nd to apply here: unfortunate.
    Wonder how/if this will apply to commercial, conservative "talk radio"?

    1. It has applied to commercial broadcasters from the start, and they've done field strength calculations (same basic thing) since the dawn of broadcasting. The only difference with hams is that we were supposed to abide by the laws, but we never had to actually put numbers on paper.

      The only hams who might have to change their stations are the guys who run more than a kW into low antennas people can get close to. The overwhelmingly vast majority will just figure a couple of ratios and be done with it.

  4. I wonder if anyone has ever been harmed from rf emissions by an Amateur Radio Transmitter ?

    All the years that have passed without this requirement placed on the Amateur Radio services........... and now this. . Something/someone drove this. An injury blamed on rf exposure perhaps ? Safety is important sure. But why now ?

    I think back to my days as a 16 yr old in High School Electronics class when I built a pulse modulator for a 250 kw x band radar magnetron coupled to a homemade horn antenna popping the flourescent tubes in the overhead lighting. Those were the days my friends. I grew up in Miami and we had 3 or 4 great surplus stores where I spent each weekend scrounging electronics gear. Great memories. And I still fathered children.

    1. I seriously doubt that anyone has been harmed, but maybe from grabbing an active antenna, not from radiated emissions? Maybe from someone running a "California Kilowatt?" Moonbounce?

      That said, I know of a local doctor, M.D., who is so concerned about WiFi that they will not work in an office with a WiFi hub like most people have at home. Those are usually less than 1 watt, and the strength drops off very fast as distance increases. Would someone like that complain to the FCC? Could be.