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Author Topic: Health Concerns and the Hygain AV-18HT  (Read 3451 times)

Posts: 10248


« Reply #15 on: January 08, 2010, 01:35:34 PM »

In the amplifier forum, there is a question about using a microwave magnetron as a transmitter. You can. But, this is one case where the power level and frequency are high enough, to boil, bake, burn, etc., so due caution is warranted. But at HF, I'd never worry unless some one physically grabbed the antenna while RF hot.


Posts: 680

« Reply #16 on: January 08, 2010, 08:17:32 PM »

Whether you think that RF exposure is a hazard or not is immaterial. The regs require you to make the assessment rather than just blindly going forward.  

Watch out for the "multiple transmitter" rule.. if you have more than one transmitter (not just amateur, that cellphone and/or wireless network count too), you have to assess them all (if only to show that the others are less than 5%).

The other thing to watch out for is "near field" vs "far field"... most of the online calculators assume a point radiator and being in the far field so they can do a simple inverse square calculation, not exactly the case with a vertical in your backyard, esp on low bands. WX7G's little calculation shows that at 70 ft (which would be the *far* side of my neighbor's lot) he's at 1/3 of the limit. Of course, I suspect he hasn't taken the duty factor and averaging into account, but still, it probably falls off as 1/r, so at 20 feet (the fence line), the level is right at the limit.

If you're thinking about it, get someone to run a NEC model, and if you're lower than a tenth of the limit (taking into account duty factor and averaging) as will almost certainly be the case, file the data in your notebook and be happy.

There was an FCC enforcement action (mentioned in QST as well) a few years ago where someone asked about an amateur's vertical antenna next door.  The FCC asked the ham for the data, but I never saw if it went anywhere after that.  Hopefully, the ham had done his assessment, eh?

Posts: 10

« Reply #17 on: January 09, 2010, 08:27:14 PM »

Did some checking and some math.  As long as you follow proper grounding, sheilding, and matching at frequencies that this antenna uses as long as your legal and over 20' away should not be a problem based on research of all the ARRL and FCC bits.  Plan to get field strengh meter out once I finish setting up mine over the next couple of weeks and document the radiation pattern and any hot spots. RF burns are possible but unlikely but I only run 100W. usually less than 20 most days. Different story for VHF-UHF anything past 50 watts needs to be carefully measured and controlled.

Posts: 729

« Reply #18 on: January 19, 2010, 02:10:54 PM »

Does this radio club "sky is falling" type also try to stifle all  radio club  attempts to accomplish something with arcane references to "Roberts Rules".  To some, fouling up other's plans is some kind of power trip.
I have had an 18 HT  up for 30 odd years, through all sorts of storms, and no three legged frogs or people have been seen nearby.   The antenna is a killer on 80 and 40, good on 20 but hears signals from all over at once (as expected) loads on 30, 17, and 12 with a tuner but is  just fair above 20.  Note the the gloom and doom type, the radiated field is directly affected by how extensive the counterpoise and ground conductivity are at the location and WHAT the antenna comprises in terms of wavelength on a particular band.  How anyone can make a blanket statement like that of Mr. Doom without knowing your power, mode, band, and counterpoise  arrangement is beyond me.  
Cnsider the number of people running a killerwatt into a "G5RV" strung to the back fence.  Are they doing the neighbors any favors?  
Put one up amd give it the "radial" system an antenna that costly deserves and enjoy it.

Posts: 7718

« Reply #19 on: January 19, 2010, 03:30:01 PM »

K1DA I agree with you. Everywhere including here at eham lurks the 'black hat' types who shoot down every idea. That is quite easy to do. We want to know how to make 'it' work, not just how 'it' won't work.

Posts: 5

« Reply #20 on: January 25, 2010, 12:13:44 AM »

I am also planning to put up an AV-18HT in my yard, which is very small.  I also had the same questions, because both of the smallness of my yard, the closeness of the neighbors, and the large amount of power that I intend to use (I am getting a 1500 W linear amplifier (actually 2500!  With an input of 65 W, it will crank out 1500, and I figure, in emergencies of life-and-death, would be most useful for really punching the signals out there with 2500 W) the Ameritron AL-1500.

This all sounded like it might be a bit dangerous, especially being a beginner and being clueless and all, but I did some research into the exact numbers and came up with the following:

1.  Using the ARRL Ham Radio License Manual (Tech entry level manual), p. 7-9 to 7-10, I calculated the AVERAGE power, which is the number that you need to plug into most calculators to get correct total numbers.  The average power is "Average power = PEP * operating duty cycle * (time transmitting / averaging period).  For example, let's say that your 100-watt transmitter was generating conversational SSB without speech processing, Table 7-4 (my note:  on p. 7-10 - this is a very important table - it shows that most of the time, especially on the HF bands, you are not using much average power) shows an operating duty cycle of 20% for that mode.  During your operating period you transmitted for 1 minute out of every three.  Your average power during the evaluation period is:  100 W * 20% for conversational SSB * (1 min / 1+2 min) = (100 * .2 * .33 = 6.33 W."  This is much lower than the initial 100 W you put in and is your AVERAGE power, which you will now use to put into an RF Safety Calculator.  

One good example of a calculator is:  I'll get back to this later, but this is the calculator I love - very simple to use and very nice.

I will also include the Table 7-4 from the ARRL book, so that you can get an idea of what the duty factor of the most common modes is:

"Table 7-4
Operating Duty Factor of Modes Commonly Used by Amateurs

Mode                        Duty Cycle      Notes

Conversational SSB          20%             1
Conversational SSB          40%             2
SSB AFSK                    100%
SSB SSTV                    100%
Voice AM, 50% modulation    50%             3
Voice AM, 100% modulation   25%
Voice AM, no modulation     100%
Voice FM                    100%
Digital FM                  100%
ATV, video portion, image   60%
Conversational CW           40%
Carrier                     100%             4

Note 1:  Includes voice characteristics and syllabic duty factor.  No speech processing.
Note 2:  Includes voice charaacteristics and syllabic duty factor.  Heavy speech processor employed.
Note 3:  Full-carrier, double-sideband modulation, referenced to PEP.  Typical for voice speech.  Can range from 25% to 100%, depending on modulation.
Note 4:  A full carrier is commonly used for tune-up purposes", ARRL Ham Radio License Manual, p. 7-10.

2.  Now for a little information from the AV-18HT manual itself.  On p. ? (pages are unnumbered), Table 1, about in the middle of the manual, different scenarios are listed as to number of radials one is willing to put in, and then the corresponding λ of each radial, and for our purposes here, especially the power gain in dB.  Depending on the number of radials one is willing to put in, the power gain can be anything from 3 to 6 (16 radials (in which the radial λ would be .1) to 120 radials (in which the radial λ would be .4))

3.  So now that you have the basic numbers, you can put them into the calculator and see how far away you must be for everything to be "safe" as it is termed.

If we take one of the most dangerous situations, it might look something like this:  

1500 W * 100% (e.g., voice AM, no modulation) * .5 (you talk 50% of the time only) = 750 W AVERAGE power.

If you put in 120 radials for 6dBi and have 100 ft. of coax, even if you use one of the best types at this λ, e.g., type 9913, you will still lose about 1 dBi per 100 ft. (taken from the ARRL book again, p. 3-17, Types of Coaxial Cable", Table 3-3.  So you would have, say 5 dBi of gain.

The frequency of operation could be, say 30 MHz, since it seems like the more dangerous HF frequencies are the higher ones.  

When I run these numbers through my calculator, here is what I get:


AVERAGE power at the antenna:  750 W
Angenna gain in dBi:  5 dBi
Distance to the area of interest: (doesns't matter what you put in; it's your guess as to how far away you will be from the antenna)
Frequency of operation 30 MHz
Are Ground Reflections Calculated?:  Check yes, if not automatically checked

Click on "Calculate RF Power Density"

In a "controlled environment" or basically your yard and property, the number I come up with is 14.30 feet, a really VERY small distance, I think.  31.92 feet is the distance to the neighbors in the "uncontrolled environment".  In the former number, the maximum permissible exposure (MPE) is measured in mW/cm2, and comes out to be 1.005 mW/cm2 in the controlled environment.  The second number, or uncontrolled environment, comes out to be .205 mW/cm2.

So, as you can see, this is probably one of the worst case scenarios.  I think 90% of what we all do on the radio will be much better cases than this.  Even if the duty cycle was 100% across the board, so that the AVERAGE power was 1500 W, the distances would still be 20 feet and 45 feet for the controlled and uncontrolled environments.

You can plug in your own numbers and come out with a more realistic scenario for yourself based on what you actually do with your own rig, mode type, how many radials you intend to put in, what type of coax cable you will use as feed line, etc.

Hope this helps.  It has definitely helped me to get some solid numbers in my mind and ease my mind about putting this type of an antenna into my small yard.  

Take care.  Any more questions, just e-mail me,  Shon Edwards, KO3U,  

73 de Shon, KO3U

Posts: 7718

« Reply #21 on: January 25, 2010, 01:25:00 PM »

And with RF if you can't see it it can't hurt you I always say.
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