Podcast: Play in new window | Download | Embed

Subscribe: Apple Podcasts | RSS

Hello all,

To listen in a new window go here BlogCast!

I am not sure if you are aware but David Horn wrote a blog at http://www.lbagroup.com/blog/fcc-contractor-rf-training/ about the mandate issued by the FCC, http://www.fcc.gov/document/verizon-pay-50k-resolve-radiofrequency-exposure-investigation%20 about Verizon Wireless getting slapped with a $50K fine. So now working there you are required to take RF Awareness training. Why? Because of RF exposure on 2 east coast rooftops. Look at the links, then come on back and I will tell you a story of someone who reached out to me about his RF exposure nightmare. Special thanks to Clifford Wilcox for sharing this on Facebook!

So, I was conversing with someone, (I will reveal his name if he would like me to but not until he explicitly gives me permission) on Facebook about his RF exposure experience. He was working close to an antenna, broadcast, and had major problems afterwards. I am going to quote exactly what he sent to me. He got ill and went to an expert to find out what the problem was, this is the report he got back from them. Let me point out that all of these problems happened in exposure that was less than 45 minutes. It was broadcast so it was high power and this is analog, which as far as I know is no longer used in the US, so things have changed.

Here is the edited report, I removed names and the identification information;

“I have gone through an analysis to try to bracket what I think you may have been exposed to. This is an estimate only! It would take considerably more time to model the batwing elements in detail. Hence, I have used a simple dipole element to estimate local fields near the element and what you might have been exposed to.

My assumptions: Frequency of exposure: NTSC analog TV signal Antenna: 12 bay batwing, Harris TAB-12H Power gain: 11.3 FCC licensed ERP for station at the time: 316 kW

Based on these assumptions, if the station were operating at FULL power of 316 kW ERP, this would imply approximately 932 watts of rms (average) power into each bay of the antenna. If the station was operating at 10% of normal power while you were working on it, the power would be, of course, one-tenth as much, or 93.2 watts per element.

I calculated the electric field strength parallel to a dipole radiator (I realize that the element is really a batwing design but that would take a lot more time to model) and found that at a distance of 1 foot from the element, the maximum electric field strength would be equivalent to a plane wave equivalent power density of about 46 milliwatts per square centimeter at full power or 4.6 milliwatts per square centimeter at one-tenth normal power. Please keep in mind that this is a value where the RF field will be maximum along the radiating element and at a distance of 1 foot from it. Since you were sitting on the element itself, the RF field that at least part of your body would have been exposed to would be much greater. I did not explore calculations at closer distances to the element since this would take additional time to make sure that the theoretical model is calculating correctly.

The accepted maximum permissible exposure (MPE) for occupational exposure in the channel 7 frequency band is 1 milliwatt per square centimeter. These results would suggest that at the presumed safe operating power level of one-tenth normal power, your exposure could easily have been as much as 4.6 times the MPE. Because you were actually sitting right on the element, I would expect your exposure to be substantially greater than my one-foot number but without more careful and detailed analysis, I can’t say exactly what it might have been.

Based on my own experience in climbing towers to make RF field measurements, I know that sometimes there can be miscommunications between the field guys and the tech controlling the station. While I have no information to suggest that this might have been the case, it is relevant to understand that were the station actually operating at its normal full power, the exposure would have been substantially greater, namely, about 46 milliwatts per square centimeter. This is, then, 46 times the MPE and would have resulted in very significant thermal loading on the body!

If the body is exposed to a uniform RF field over the whole body equal to 4.6 times the MPE, the thermal load imposed on the body could be about 130 watts. This power would be distributed throughout the body and while above the accepted limit of about 28 watts (equivalent to exposure at the MPE), would likely have been felt but, in my opinion, not necessarily hazardous. I say this because the MPE has a built-in safety factor of 10 in it. But, if you were exposed to the full power of the station, the thermal load on your body could have been in the neighborhood of 1,300 watts! This is a completely unacceptable value and would be almost five times the hazard threshold (i.e., the hazard threshold can be thought of as a power deposition of about 10 x 28 watts or about 280 watts – in other words, the safety factor of ten has been removed and we are now right at the hazard threshold).

The 1,300 watt thermal load figure is approximately 12 times the body’s basal metabolic rate of 105 watts (i.e., the thermal generation of your body just sitting still). This kind of thermal loading would have been distinctly felt by you as getting hot! It is my understanding that you were told that the station was operating at the so-called safe level of only 1/10th normal power. And, if this is true, then your exposure, while still above regulatory limits, might not have been sufficient to result in a hazard. But, IF, by chance, the station was operating at full power, your exposure would have not only been way, way above regulatory limits, you would have, in my opinion, been sort of cooked. I am only trying to give you some perspective on your possible exposure. Apparently, we don’t really know for absolute sure what power the station was operating at. A first start would be to get copies of the station’s log book during the time you were working on the tower to see if there is validation of what they told you about what power they were really operating at.

In summary, even at the one-tenth power operation, I think that there is a high likelihood that you would have been exposed to RF fields exceeding the worker MPE, though it might have not resulted in reaching the actual hazard level of energy absorption rate. On the other hand, if the station were to have operated at full power, your exposure would have not only exceeded the worker MPE, but it would have likely resulted in significant body heating, well beyond the level that is believed to be hazardous. Because you were exposed while sitting on the top batwing for at least 30 minutes, the issue of time-averaging is not relevant.

One other point. The exposure limits are based on limiting the rate of energy absorption in the body as a whole, what I have been discussing above, and in local parts of the body. You can think of this as though there is a power deposition limit as averaged over the whole body mass as well as a power deposition limit for any given specific point in the body. I have only addressed the case of the average over the whole body in this analysis. The issue of what the localized RF absorption rate may have been in various parts of your body is another question.

I hope this provides useful information to you in regard to your exposure while working on the tower”

USA TV broadcast frequencies [http://en.wikipedia.org/wiki/North_American_television_frequencies]

OK, there are a lot of “what-ifs” in this report but it doesn’t change the fact we all need to be educated and aware. So RF awareness is something we all need! You need to be aware of what is on the tower. I say it all the time, there are more risks than the fall. Wake up people! We need to work together to provide a safe work environment. Let’s learn from these mistakes and make it a safer future for all wireless field workers. Awareness is the first step! Jimi Hendrix asked, “Are you experienced?” [ https://www.youtube.com/watch?v=zg2segLZoeA ]so I am asking you, “Are you aware?” [I don’t have a video, sorry]. Aware of the risks out there beyond the fall. Wake up and be aware! This is a brotherhood and a team, together we learn and teach each other. It’s more that climbers, it’s the wireless field workers that need to work together.

After I put this together I feel we should consider having everyone wear RF alert monitors while working, especially on rooftops. I would only wear them at broadcast sites if I climbed, but I am thinking we should make them standard equipment for all workers at the tower site. Let me know what you think.

OK, shameless plug time. My new book is out, I create a Wireless Field Worker’s Aid for Tower Site Work

I am working on getting out on Amazon, not quite there yet.

PDF on Gumroad (Credit Card) https://gumroad.com/l/RSJZ

PDF on Sellfy (PayPal) https://sellfy.com/p/kxAw/

If you want to reach out to me, Facebook is the best way, Feel free to Twitter or Google+ or email wade4wireless@gmail.com to reach me as well. Or leave a comment on this blog.