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Don’t Cook Yourself With RF Energy

I’m getting ready to install a radio antenna on my home’s roof, and hoping to avoid hurting myself or anybody else in the process. While falling off the roof may be the most obvious risk, a less obvious concern is the radio frequency energy from the antenna when it’s transmitting. Is this something to worry about it? How much RF exposure is too much? Let’s take a look.

Electromagnetic waves in the radio frequency spectrum are what’s called non-ionizing radiation. They don’t have anywhere near enough energy to knock electrons loose from an atom or molecule. Unlike x-rays or gamma rays, they can’t cause radiation sickness, genetic damage, or cancer. The primary human risk from RF is heating up tissue – it can cause burns or literally cook you. This is how a microwave oven works, for example. There’s no risk for a human near an antenna that’s only receiving, but you need to be careful any time the antenna is transmitting. The risk depends on several factors, some obvious ones and some less obvious:

  • Transmitter power – Higher power means higher RF energy levels.
  • Distance from the antenna – The closer you are to the antenna, the higher your exposure.
  • Transmitter duty cycle – Exposure is averaged over a period of several minutes. A 100 watt continuous transmission creates equivalent exposure to a 200 watt transmission alternating 1 minute on, 1 minute off.
  • Height above ground – Radio waves can bounce off the ground and reflect back up, concentrating more of the total energy in a single spot.
  • Proximity to buildings and nearby objects – Radio waves can also bounce off buildings, creating a similar effect to ground bounce.
  • Antenna type – RF energy isn’t radiated equally in all directions. Some antenna types concentrate more energy at low elevation angles close to the horizon, or in specific directions. A human in the high-concentration zone will get higher RF exposure.
  • Transmission frequency – The human body is most efficient at absorbing RF energy in the 30-300 MHz frequency range, so the risk is higher for radio transmissions in this range.
  • Transmission mode: voice, digital, or Morse code – A 100W-rated radio that’s continuously transmitting may not be outputting 100W continuously; it depends on the transmitted signal. Continuous wave (CW or Morse Code) transmissions come closest to reaching the advertised power all the time. FM voice does too. But AM voice and single sideband (SSB, a type of AM) transmissions have an instantaneous output power that depends on how loudly you’re speaking at that moment. If you’re silent, the power is effectively zero.

In the United States, the FCC’s OET Bulletin 65 Supplement B describes the maximum permissible exposure or MPE limits for RF electromagnetic fields, and some methods used to calculate the exposure. MPE limits are defined in terms of power density (units of milliwatts per centimeter squared: mW/cm 2), electric field strength (units of volts per meter: V/m) and magnetic field strength (units of amperes per meter: A/m). Fortunately you don’t need to understand the details of all this, and can simply use their data tables to determine how far away people must be from the antenna in order to stay safe.

Controlled and Uncontrolled Exposure

The FCC sets two different MPE limits, for controlled and uncontrolled exposure. The controlled limits are higher, and they apply when people know they’re close to an antenna and have been trained to understand the potential risks. Typically this would mean a workplace setting, but controlled exposure limits also apply for licensed amateur radio operators and their families, assuming they’ve had RF safety training. The uncontrolled exposure limits apply to the general population, who may be completely unaware of the risks or who don’t know there’s an antenna present.

I don’t understand the reasoning behind this. Just because someone knows there’s an antenna, and has received RF safety training, how does that make their body less susceptible to injury from RF energy? If you can explain this, please do. For my home, I’m choosing to use the stricter uncontrolled general population exposure limits even though I’m not required to.

Doing the Math

Let’s look at some specific numbers. I’m installing a dual-band VHF/UHF antenna that’s a clone of the Diamond X30. It transmits at about 145 MHz for VHF, or about 450 MHz for UHF. It’s a vertical antenna with an omnidirectional radiation pattern in azimuth, but the pattern is squished vertically. More energy is radiated straight out toward the horizon, and less energy up or down where there’s usually nobody to talk to anyway. The radiation pattern is like a flattened donut, with the antenna centered at the hole in the middle.

This particular antenna has a gain of 3.0 dBi for VHF and 5.5 dBi for UHF. This gain isn’t true amplification, it’s just a measure of how concentrated the RF energy is in the direction of interest relative to a hypothetical reference antenna. The “gain” comes from the degree of squishing in that radiation donut.

The radio uses standard FM modulation, and has a maximum output power of 25 watts, although most of the time I’ll probably run it at lower power.

Marshalling all this information, we could use the data tables in the FCC bulletin to estimate the minimum safe distance from the antenna for uncontrolled exposure. Fortunately several people have created easy-to-use calculators based on the FCC data, so let’s use this RF exposure calculator from Paul Evans VP9KF. Plugging in 25 watts, 3 dB gain, 3 meter distance, 145 MHz, with ground reflection, the calculator tells us 3 meters is safe and 2.27 meters (7.45 feet) would be the minimum distance. With 5.5 dB gain at 450 MHz, the safe distance is 2.47 meters (8.1 feet).

Going the Distance

8.1 feet isn’t a huge distance, but neither is it small. I probably don’t want to put that antenna directly outside the window from where I’ll operate the radio, or on the roof directly overhead that spot. Nor should it be anywhere that somebody might come and sit within 8 feet of it for an extended period of time. This might not be easy to achieve.

It’s important to understand that this 8.1 feet number is the most pessimistic and conservative number. It assumes my radio is continuously transmitting 100 percent of the time, which it certainly won’t be. It assumes the 25W from the radio is completely radiated as RF energy, when in fact some energy will be lost in the transmission cable to the antenna. It assumes the worst possible ground bounce. And it assumes a person is in the main lobe of the antenna’s radiation pattern, where the energy is concentrated. For this particular antenna, if it’s on the roof and people are below it, their RF exposure will be less because the antenna radiates most energy horizontally rather than up and down.

My planned location for the antenna is on a mast, mounted above the second story roof, above my garage. Yes my garage is on the second story of my home. Ideally the mast would be as tall as possible, but to minimize the visual impact and possible neighbor complaints, I’ll be using a very short mast – about 1 foot. If someone were standing at the back of my garage for an extended period of time, they’d be about 5 feet below the antenna. This might be a small concern, although it still meets the MPE for controlled exposure even before considering all the mitigating factors just mentioned. Outside the garage, it’s impossible to get closer than about 13 feet to the antenna unless you’re standing on the roof. We should be fine.

Read 11 comments and join the conversation 

11 Comments so far

  1. Leon Theremin - April 14th, 2022 12:27 am

    Meanwhile, every cell phone transmitter is a hacked microwave gun with unlimited ammo firing at children all day.

    Watch this video titled “Defeating Microwave Weapons!” and learn how to defend yourself against them.

  2. Snarl Broodlesnap - April 14th, 2022 2:29 am

    “…they [non-ionizing] can’t cause radiation sickness, genetic damage, or cancer”.

    Well, yes they can. Just not directly as with ionization, but via a cascade of cell-membrane voltage-gated channel mediated biological effects leading to intracellular overproduction of reactive oxygen species (free radicals). See this:

    Modulated signals more bioactive than continuous wave.

    Take care.

  3. Rusty Shackleford - April 14th, 2022 5:18 am

    No sense in parroting this drivel.

  4. tcp1 - April 14th, 2022 5:41 am

    Yes, @leon Theremin, and there’s also this thing that’s a “photon gun with unlimited ammo” that kids point at each other all the time. They’re called flashlights.

    People like you are so tiring.

    We’ve been working with radio for over a hundred years now. We understand it pretty well. There are no disease clusters in ham radio operators or people that use radios near their body all the time – like cops or other first responders.

    Keep trying to go after a boogeyman because you don’t understand something, though.

  5. Steve - April 14th, 2022 7:38 am

    Oh boy, here we go. There are real risks from high levels of RF, which is what I’m concerned with in this post. As for low-intensity RF, I try to keep an open mind and consider all the evidence, even from sources that may be out of favor. I did read through the paper linked above. But if somebody’s looking for a “tear down all the 5G towers” guy, that’s not me.

  6. Steve - April 14th, 2022 10:38 am

    Let me elaborate on this. I’m mainly concerned with high-intensity RF that can cause thermal heating. That’s a clear risk that the FCC requires amateur radio operators to consider and plan for, and the topic is also covered in the license exam question pool. From my analysis here, my home antenna should be fine.

    It seems plausible that low-intensity RF could interact with biological processes, and that some of these interactions could be disruptive or harmful. It’s a topic I’ll keep an eye on, but I’m not losing sleep over it. Yes you’ve got oscillating electromagnetic fields in the body, at frequencies that don’t occur naturally. They’ll be weak fields, but the human body is a low-voltage device. “What about flashlights” doesn’t seem like a good counter-argument, because the frequency of visible light is a million times higher than typical RF. Radio frequencies are closer to the natural resonant frequency of biological molecules. This paper’s presence on ResearchGate doesn’t bother me – lots of papers are shared there. It was originally published in the journal Electromagnetic Biology and Medicine, which looks at least halfway credible, based on a few minutes of skimming it.

    But having said all that, what level of real-world biological risk might exist? How does it compare to the risks of air pollution, Covid-19, microplastics, burnt meat, traffic accidents, or lightning strikes? I agree with tcp1 that it would be more compelling to see a study showing significantly more health problems among people who work or live near low-intensity RF sources. I’m not aware of any such data. Because I’m not an expert in these fields, I look to the FCC or other government sources to define safe RF exposure levels, and so far they haven’t set any limits for low-intensity RF.

    In short I’m not discounting there might be some risk from low-intensity RF, but I’m not convinced it’s a significant enough risk to worry about. I’ll reserve the right to change my mind in the future. 🙂

  7. Snarl Broodlesnap - April 15th, 2022 2:07 am

    Thanks for your considered reply.

    Regarding your comment:

    “…be more compelling to see a study showing significantly more health problems among people who work or live near low-intensity RF sources. I’m not aware of any such data.”

    You may further consider this literature review:
    “Epidemiological Evidence for a Health Risk from Mobile Phone Base Stations”
    (original journal source)

    TLDR; “..We found that eight of the 10 studies reported increased prevalence of adverse neurobehavioral symptoms or cancer in populations living at distances < 500 meters from base stations. None of the studies reported exposure above accepted international guidelines"

  8. Lee Adamson - April 17th, 2022 8:28 am

    Unless you are running some real QRO, I don’t think you have anything to worry about.

    1. Your 144MHz signals will be 50 watts or less.
    2. 144MHz causes far less tissue heating than microwave signals.
    3. Your duty cycle will be fairly low. Even if you start running digital modes it will be 50% or less.
    4. If you go general and get into HF, even less tissue heating at those frequencies, even at 100-150 watts.

    I wouldn’t want to hang from the feed point of an antenna being fed with 1.5kW, nor sit on a microwave dish all day, but if you are a few feet away your 50w VHF or 100w HF rigs aren’t enough danger to worry about, imo.

  9. Steve - April 18th, 2022 3:10 pm

    Yes, this is my inexperience making me more cautious than is probably necessary. I’m going exactly by the book for the RF exposure calculations as they were outlined in the licensing test, and getting a “most conservative” distance. If I can maintain that distance, I’m good. If I can’t maintain that distance, I’m probably still good anyway, but I need to do a little more analysis for duty cycle and some of the other factors you mentioned.

  10. Dan+Draper - April 20th, 2022 5:43 pm

    This is very interesting but what I really want to hear more about is your garage on the second floor of your house

  11. Steve - April 20th, 2022 5:57 pm

    Haha yes. Actually the garage is on the THIRD floor, if you count the lowest floor as the first. There’s a steep downslope involved, and the yard and most of the house are below the street level. The garage is at street level, above the rest of the house.

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