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Author Topic: RF Radiation Danger?  (Read 9664 times)
KK4AXX
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« on: July 09, 2012, 11:12:09 PM »

Here's the set-up:

I work a 100W rig.  Sometimes through a 40M Loop Skywire, but more often through a pair of ground-mounted verticals - a home-brewed copper pipe 17M vertical and a re-built 4BTV.  All antennas work great and I'm more than happy with them.

Here's the thing, and perhaps I'm being overly cautious... I'm also the father of six kids, four of whom are still at home.  We also have a pair of Great Danes, a Shih Tzu, and the dumbest Cocker Spaniel I've ever seen.  Generally I'm not transmitting when they are out in the backyard, but sometimes I am until they come into my line of sight.  With only a 100W transmitter, what is the expected RF exposure?  I'm assuming it is minimal.  The kids all know NOT to touch the vertical antennas, but the dogs are likely to stick their noses to anything out back.  So, what are the potential hazards with direct contact with a transmitting vertical antenna at a maximum power of 100W?

It would be nice if I could add a LED light out there that would light up automatically on transmit.  Any ideas how that could be done?
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George U. Potter Lodge, #912, F. & A. M. of Alabama
A.A.S.R., S.J. - Valley of Mobile, Orient of Alabama
Dave Langham Chapter, #536, Order of the Eastern Star
Order of DeMolay, Mobile Chapter, Adult Advisory Council
AF6WL
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« Reply #1 on: July 10, 2012, 01:10:58 AM »

How about a fluorescent tube : http://www.youtube.com/watch?v=SDL9O2Fwb64
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WX7G
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« Reply #2 on: July 10, 2012, 01:14:10 AM »

This RF Safety calculator will help you decide: http://hintlink.com/power_density.htm

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KK4AXX
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« Reply #3 on: July 10, 2012, 02:51:20 AM »

Thanks for the link to the calculator.  That answered several unspoken questions.  Still wonder what physical contact would do to a luckless dog's nose... or some kids hand.

As for the light, a small florescent bulb *might* work, but remember I'm only running 100W maximum.  The verticals are mounted on a 4x4 inch wooden post with 4 feet above ground.  It would be neat to have a red LED light on top of that post that would glow during transmission.  The circuit would be no farther than five inches from either antenna.  Making the entire exercise even more pleasant is that it would be clearly visible from my chair.
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George U. Potter Lodge, #912, F. & A. M. of Alabama
A.A.S.R., S.J. - Valley of Mobile, Orient of Alabama
Dave Langham Chapter, #536, Order of the Eastern Star
Order of DeMolay, Mobile Chapter, Adult Advisory Council
G8HQP
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Posts: 123




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« Reply #4 on: July 10, 2012, 03:38:03 AM »

Anyone standing next to the antenna is in the near field, so exposure depends to some extent on the antenna Q. Narrow bandwidth antennas, as typically used by amateurs, have stronger local induction fields than a genuine wideband antenna for the same radiated power.

You can estimate the voltage experienced by someone touching the antenna. An ideal resonant grounded quarter-wave would have an impedance of about 35 ohms. At 100W this is a peak voltage of 83.66V. Even DC at this level would be enough to make someone jump. AC and RF would be worse. A real antenna would be worse, as any reactance seen at the base would increase the feed voltage. Lets make a wild guess that at one of your operating frequencies the antenna looks like 20+100j. 100W then means peak voltage of 322.5V. My guess is a nasty slow-to-heal RF burn.
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K4SAV
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« Reply #5 on: July 10, 2012, 07:19:17 AM »

G8HQP is correct and the numbers can be worse than what he gave you the calculations for.  He calculated the voltage at the bottom of the antenna.  The voltage gets higher at higher levels up on the antenna.  There can be enough to cause a nasty burn and those take a lot longer to heal than a heat burn.  You need to protect this.

I was going to suggest that you put a PCV pipe over the antenna up to the point that no one can touch it, however if it is ground mounted and supported by a 4x4 pole, that might not work.  You could get a piece of perforated drain pipe, split it if necessary, and place it over the vertical and the post.

I don't think those RF safety calculators work very well for near field calculations.  EZNEC can be used to calculate the fields at any position near the antenna, and you can compare the results to the limits stated in this document:
http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet65/oet65b.pdf

Here is an example:  For operation on 10 meters the limit for the E field is 824/freq V/m, and the limit for the H field is 2.19/freq A/m.  So for 28.5 MHz the limit is 28.9 V/m E field and 0.077 A/m H field.  You get to derate the power level based on the duty cycle of operation.  The referenced document gives some guidelines for derating the numbers.  I don't know how you operate so I will give you a number based on continuous operation at 50 watts. 

The worst case for the E field happens higher up on the antenna.  A 6 ft tall person would have be exposed to much greater fields than a short person.  At 6 ft height the E field limit is exceeded at about 4.5 ft. from the antenna (remember that number is for continuous operation at 50 watts).  The H field is greatest at the lower portion of the antenna and the limit is exceeded at about 6.5 ft from the antenna and 1.5 ft above ground (again for continuous operation at 50 watts).

Every time this subject comes up there are those that appear and tell you to ignore the RF safety rules because there is no proof it causes damage.  It doesn't matter what you believe.  As amateurs, the rules say we must abide by the numbers in that document.

Jerry, K4SAV
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N4UM
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« Reply #6 on: July 10, 2012, 07:50:08 AM »

Why not get some of that plastic fencing and place it around the verticals so that the kids and dogs can't even get close to the antennas?
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KK4AXX
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« Reply #7 on: July 10, 2012, 09:18:24 AM »

Oh yeah, I'm definitely gonna have to fence off that post!

...and if I mulch the entire fenced off portion it will be that much less grass I have to cut! Grin
(Well, that much less my sons have to cut, but you get the point.)
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George U. Potter Lodge, #912, F. & A. M. of Alabama
A.A.S.R., S.J. - Valley of Mobile, Orient of Alabama
Dave Langham Chapter, #536, Order of the Eastern Star
Order of DeMolay, Mobile Chapter, Adult Advisory Council
KB2WIG
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« Reply #8 on: July 10, 2012, 09:48:11 AM »

Is there any way you can slip a 6' piece of plastic pipe over yer verts?? Its a bit easier than putting up fencing.



klc
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K8AXW
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« Reply #9 on: July 10, 2012, 10:08:00 AM »

Quote
Why not get some of that plastic fencing and place it around the verticals so that the kids and dogs can't even get close to the antennas? 

Best answer.  As another stated, your kids and dogs can get a nasty RF burn if they touch the antenna.  Since you will no doubt be using CW or SSB there isn't much chance of getting "hung up" on the antenna.  But getting burned is real.

I pay no attention to this RF filed BS that is the result of one man's 15 minutes of fame like the idiot who was able to bring the world to using the word Hertz instead of cycle.



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K4SAV
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« Reply #10 on: July 10, 2012, 12:51:11 PM »

Not that you need it for amateur radio purposes (FCC rules specify field levels which you have to maintain, not that you can't cook someone), but if you want some information on the effects of RF exposure on the human body, read the comments by W6RMK in this thread.  The real meat starts on page 2.
http://www.eham.net/ehamforum/smf/index.php?topic=81271.0

Jerry, K4SAV
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KD8MJR
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« Reply #11 on: July 10, 2012, 02:08:24 PM »

I don't know what to think of all of this but I will say that Ham Radio is one of 4 hobbies that I am involved in and my Wife's is in the Medical field.   With that said I have never ever heard of more people that have Cancer than I have since I became a Ham.  Almost everynight I turn on the Radio and within 20 minutes I hit up onto a "I got cancer conversation". That's very disturbing and while I am not directly linking this to Ham Radio I certainly would not just dismiss it out of hand.  I know a lot of old people from my wife's work and many are between 70 and 90 and maybe only 1 or 2 out of 10 ever seem to get cancer but wow on HF it sure seems like a lot more people get it judging by the amount of conversations.
Is it that people just talk about it more on Radio?   Sounds strange to me since you know the world is listening.  So as I said i don't know what to make of it but I do know I only clip on my Amp when I need to.

BTW did any of you see the Larry King special on Cell Phones and Cancer?  It was based on the worlds largest study over a 20+ year period using of thousands of participants.  It aired just one night on one time slot and has never been seen again.
 
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K7MH
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« Reply #12 on: July 11, 2012, 11:55:50 PM »


Quote
Sounds strange to me since you know the world is listening.
Not really. I guess you haven't spent too much time on 75 phone!! Grin

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KK4AXX
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« Reply #13 on: July 13, 2012, 10:55:14 PM »

I am even more determined now to place a "Transmitting!" warning light out there.  As I said, I can see the verticals from my radio.  Kids, however, will be kids and I can't vouch for the neighborhood children that come over to play. 

These verticals are mounted on the same post with a 10 inch separation.  (Yes, there is some debate whether they should work when that close together, but after mounting the second (a 4BTV) only minor tuning of the 17M antenna and the 20M trap on the 4BTV was required.)  Perhaps I can locate a length of the large ABS drain pipe that will fit over both at once, though a small fence may be more eye-appealing...
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George U. Potter Lodge, #912, F. & A. M. of Alabama
A.A.S.R., S.J. - Valley of Mobile, Orient of Alabama
Dave Langham Chapter, #536, Order of the Eastern Star
Order of DeMolay, Mobile Chapter, Adult Advisory Council
G3RZP
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Posts: 4552




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« Reply #14 on: July 14, 2012, 01:48:57 AM »

Be careful if you use RF to activate LEDS - you can cause harmonic problems. The best bet is a neon lamp or a flourescent.
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