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Author Topic: FCC seeks to reassess RF exposure limits  (Read 23848 times)
W6RMK
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Posts: 651




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« Reply #15 on: July 16, 2013, 09:35:33 PM »

What about the typical mobile user such as myself running on 50W VHF when we're having trouble working the repeater on lower power or seeing what we can work simplex? Under the current rules I don't have to evaluate, but the suggested example of 24 feet under the new example would be nowhere near possible in my compact car.


The push for the assessing RF exposure limits are on because of the nature of litigation in this country now--something driven by lawyers, not scientists and engineers.  If it WERE up to the scientists and engineers, this reassessment wouldn't be done, simply because the scientists and engineers know that the current levels are more than adequate now.

As an engineer who does these sorts of analyses as part of my job, I think a bit of clarification is in order.  The issue here isn't the levels of RF exposure: as noted, they've got good scientific justification and epidemiology behind them.  It's whether the existing safe harbor that lets you avoid the analysis is appropriate. As noted in the FCC document, the safe harbor for VHF isn't particularly practical for a mobile installation.  You're a lot closer than 24 feet. So, under the current rules, you need to do an analysis.  For a lot of hams, the duty cycle averaging allowed will mean that even though you are only a few feet from the antenna, you're still under the maximum permissible exposure.

But you need to do the analysis to be sure.  If you're running something with high duty cycle (ATV repeater? APRS in some weird mode?) you need to deal with it appropriately.  If you're contesting from your car (where you tend to have higher than average duty cycle) or running JT65 moonbounce or something, maybe you bust the limit.  You don't know unless you do the analysis.

The whole near field exposure is another can of worms. I'd hate to depend on some sort of point source approximation to calculate fields for a backseat passenger in a car with a trunk lid mounted antenna. For OET Bulletin 65, they took some typical ham installations and measured/analyzed them in detail to formulate the safe harbor limits. Maybe the same needs to be done with sort of canonical 2m or 70cm mobile installations.
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K1CJS
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« Reply #16 on: July 19, 2013, 04:58:42 AM »

Lets get one thing straightened out.  Those so called 'limits' were enacted in the last decade or so.  Before that, there wasn't any need for limits since lawyers hadn't 'infected' everyone with their ideas of "If you get hurt--sue!"  The absolute worst thing our society did was to let lawyers advertize their services.

There wasn't any reported incidents of anyone getting hurt (outside of touching an antenna and getting shocked) by RF exposure, and there were people constantly exposed to the RF of VHF and UHF transmitters in vehicles--with antennas on the trunk lids or close to the operators of the transmitters.  Nobody got sick, nobody got themselves 'cooked', nobody got hurt!

It is totally ridiculous how RF exposure now has everyone up in arms--over a cellphone putting out less than a watt, or by radio tranmsmitters putting out 50 to 100 watts!  Broadcast stations put out thousands of watts at their transmitters.  Still nobody around those transmitters hurt!  Its totally ridiculous to think that people are now going to get hurt--when radio transmitters have been around for years and nobody was hurt then!
« Last Edit: July 19, 2013, 05:01:24 AM by K1CJS » Logged
WX7G
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« Reply #17 on: July 19, 2013, 01:34:53 PM »

RF exposure standards and limits are not new.

The first ANSI RF Exposure standard in the U.S. was issued in 1966 with a limit of 10 mW per square cm. These standards are now handled by the IEEE and are reviewed every few years. As knowledge of human physiology as it relates to RF exposure evolves it is appropriate that the standards evolve also.   
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W6RMK
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« Reply #18 on: July 20, 2013, 08:19:44 AM »

10mW/cm^2 is only at some frequencies in the VHF, low microwave area.  the exposure limit is higher at lower and higher frequencies because you don't absorb as much energy from the field.

The limits are, for the most part, based on heating effects and related to Specific Absorption Rate (SAR).. how many joules per gram or cc of tissue, and then relating that to a temperature rise and that in turn to biological damage due to the rise. The temperature at which tissue damage occurs is surprisingly low (a few degrees C above normal body temp of 37C.. if you're running a high fever, tissue is being damaged).

The most recent revision of IEEE C95.1 had substantial changes, not so much in the levels, but in the terminology.  It's no longer "controlled and uncontrolled" exposures, but Maximum Permissible Exposure (MPE) for occupational exposure, and an "action level" for the general public.  This is to harmonize it with other safety standards, including the ICNRP, used in other parts of the world.

There's also a lot of work on trying to understand and appropriately describe limits from localized sources (e.g. a cellphone held next to your head). Historically, the limits were sort of "you're in a uniform field of this intensity" or for localized sources (diathermy or electrocautery).  The problem is that this isn't a very good model: different parts of your body absorb differently, different parts of your body have different damage thresholds, and the incident energy is highly non uniform. That's why the limits for cellphones are not "radiated field" but are SAR, where you have to calculate or measure the actual energy absorbed (often using a RF similar phantom head with probes)


Note that the levels are, by design, conservative.  They're basically set at 1/10th of the lowest level at which any effect has been reported in the research literature.  And, remember, it's based on tissue damage as the effect.  One can damage tissue and it recovers, generally with no long term effects. So you could in some cases receive substantially over the limit, take the damage (which you might not even be aware of), and heal.  The real question is whether the "long term effects" are fully understood. Consider light sunburns: the tissue is slightly damaged, it recovers.  Do it a lot, though, and there's an increased incidence of skin cancer.

I'm not talking, here, about potential long term effects from low exposures (below the damage level). I don't think there are any, at least in a statistically measureable sense.  If my body got heated up slightly due to RF, how would that be distinguishable from me being out in the sun (an incident flux of 70 mW/cm^2) or running a fever?


there's also the disturbing issue of the studies showing increased incidence of gliomas (a type of benign brain lesion) in cellphone users.   While in general, I don't think there's any athermal effects from RF, I wouldn't be sleeping with my cellphone strapped to my head, either.
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