In studying for the exam, one question came up about RF Exposure.

What effect does transmitter duty cycle have when evaluating RF exposure?

My understanding of Radio is that it is non ionizing electromagnetic radiation. How can RF be damaging?

  • $\begingroup$ Preliminary additional answer: During the cold war, Russian scientist also ran clinical tests to prove microwaves can induce headaches. I will try to locate references. $\endgroup$
    – on4aa
    Nov 12 '13 at 18:06
  • $\begingroup$ According to the "Safety Code 6" regulations in Canada, equivalent in regulatory purpose to the FCC's regs on RF exposure, the primary health and welfare concern is damage to people's eyes by very high levels of HF frequency band RF, below 30 Mhz. hc-sc.gc.ca/ewh-semt/pubs/radiation/… $\endgroup$ Feb 25 '14 at 20:39

The direct effects of RF on people are:

  1. Tissue Heating
  2. Electric shock (shocks and burns) and electrocution (death)
  3. Interference with implanted medical devices

The General test question quoted is about evaluating the exposure for the purposes of tissue heating. I assume everyone "gets" not grabbing a wire you are pumping 1500W into.

Tissue heating is a problem because unlike being in a hot place, where the heat is outside the body, the heating is internal. The only mechanism for the body to deal with this is blood flood, so areas with little flow (for instance, the eyes) are particularly vulnerable.

To give you an idea of how you absorb the RF, consider this: the human body, standing and somewhat grounded, is a 1/4 wave vertical antenna, resonant at 4x your height. But is somewhat lossy and high impedance compared to a metal antenna. Of course, that high impedance is the problem, the RF is "lost" as heat, in you.

Safety experts consider the most the human body can tolerate is 4W/kg of heating. To put this in perspective, you generate about 1W/kg sleeping, 2-3W/kg in heavy exercise. Exceed this and you body has trouble dumping the extra heat and the core temperate begins to rise. Cell death begins to occur at 107 deg F (that's why 105 def F is a tripwire in fevers). Natually you can tolerate some of this. Depending on for how long and what's getting heated. I don't recommend some organs like the brain, liver, kidneys, things you occasionally need.

A somewhat wordy reference can be found here: http://hps.org/hpspublications/articles/rfradiation.html

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    $\begingroup$ And by "burn" they mean a real burn, much as if you'd grabbed something that was very hot if your hand were to suddenly become the ground path between your rig and its ground when it was transmitting. $\endgroup$ Nov 6 '13 at 19:51

Heating; it can burn you the same way a microwave cooks food, or UV from the sun gives you sun burn. The wikipedia article about radiation burns generally covers RF a bit: http://en.wikipedia.org/wiki/Radiation_burn

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    $\begingroup$ Consider expanding your answer a little bit - perhaps quote part of the relevant portion of the Wikipedia article, or paraphrase further. $\endgroup$
    – Amber
    Nov 6 '13 at 17:52

There is a government doctrine on this very subject entitled OET-65.

You must remember that RF exposure is cumulative and does the most damage over a long period of time. The symptoms of over-exposure to RF are stomach pains, scratchy/sandy feeling eyes, the internal feeling of overheating (possibly followed by stroke). The most effective ways of avoiding the effects of RF exposure are limiting your exposure times, RF reflective clothing and wearing an exposure badge.

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    $\begingroup$ RF Reflective clothing....? Does that include Al foil hats? $\endgroup$
    – Paul
    Nov 9 '13 at 4:28
  • $\begingroup$ When you refer to specific regulations/guidelines/etc, it's helpful to note which jurisdiction you are talking about. In this case the document OET-65 is an FCC item and therefore applies to the US. $\endgroup$
    – WPrecht
    Nov 9 '13 at 21:27

The best, and most extreme, example to answer your question would be a microwave oven. You're applying RF radiation to your food and it gets heated as a result. Tissue damage can occur when your body isn't able to dissipate the heat generated by absorbing the RF radiation quickly enough.

  • $\begingroup$ So why do they need to measure how much exposure you are getting? If you start feeling hot while standing next to an antenna , walk away, problem solved. $\endgroup$
    – spuder
    Nov 6 '13 at 18:03
  • $\begingroup$ @spuder - innocent bystanders might not realize what's going on. And there are regulations about how much RF power you can expose other people to, so you have to know how to figure out what your station is doing. $\endgroup$
    – Pete NU9W
    Nov 6 '13 at 18:06
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    $\begingroup$ Also the part that gets heated may be inside your body and without nerves. $\endgroup$ Nov 6 '13 at 18:10
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    $\begingroup$ Spuder, if it's already hot outside you may not be aware that you're also being heated by absorbed RF. According to the Health Physics Society, the eyes and testes are most vulnerable to RF heating so I think it's much better to be proactive than possibly blind and/or sterile. $\endgroup$
    – Scruffy
    Nov 6 '13 at 19:12

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