I haven't tried to calculate RF exposure data this way but you can get
near field data from EZNEC.
Maybe just use a generic 3 element 10 meter yagi. Something like
0 -8.39 50 0 8.39 50 1 11
6.88 -8 50 6.88 8 50 1 11
13.7 -7.9 50 13.7 7.9 50 1 11
Change the height to whatever you have.
Then use the "Setup/Near Field" menu to set up the areas of the near
field you want to see.
Set the power level under "Options/Power Level".
Then Select the NF Tab to see a tabulated table of data. Notice the
disclaimer at the top of this table.
Use the Gov pdf document Jim Lux referenced to apply this data.
Jerry, K4SAV
Jim Lux wrote:
>At 04:56 PM 4/24/2006, Bill Tippett wrote:
>
>
>>N6KJ wrote:
>>
>>
>>
>>>Actually, what I'm trying to do is figure out the radiation pattern on
>>>the ground. Why? I'm doing RF Exposure calculations and rather than
>>>assume the worst case exposure on the ground based upon the gain of
>>>the antenna, I'd like to know what the actual gain is below the
>>>antenna on the ground. I'm assuming it will be considerably less.
>>>For example, what is the gain of my antenna at a 45 degree angle in
>>>front of my yagi, but on the ground?
>>>
>>>
>> OK I understand now. Here is what I
>>would use if I were you:
>>
>>http://n5xu.ae.utexas.edu/rfsafety/
>>
>> 73, Bill W4ZV
>>
>>
>
>That web calculator implements some of the equations described in OET65
>Supp B, but you really need to look at the whole bulletin to understand the
>assumptions. The web calculator assumes the far field approximation, but
>also assumes you're in the main beam, so, *in general* it's an overestimate.
>
>Considering you're probably in the near field of the antenna, you need to
>take that into account. As a rough estimate, if you're within (2 pi) 6.28
>wavelengths, you should at least consider near field effects. You could
>either use something like NEC to calculate the field strengths, or read
>through the stuff in OET Bulletin 65 Supp B, and use the assumptions and
>equations there.
>
>And, if you want to be thorough, you should probably also consider that
>the SteppIR elements might be in a configuration which pushes more power
>down (although I'm not sure such a configuration exists..) In any case,
>you might be reasonable to consider a worst case as putting all the power
>into a dipole at the relevant height. Look at the table on page 32
>of OET65B for the limits.
>
>http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet65/oet65b.pdf
>
>
>If you do the dipole calculation, and you're good to go by, say, a factor
>of 10, I wouldn't bother doing anything more, unless there's some weird
>situation (like your beam points into a neighbor's house that's above you
>on the hill).
>
>Since this is a safety survey, you also need to consider "off nominal"
>situations, and at least put that into your report. Here's some ones to
>consider:
>
>1) If you have a crankup tower, what's the exposure when the tower is
>cranked down? if it's too high, you can put a note into your report saying
>you have a procedure to verify that the tower is up before applying full power.
>
>2) What if the antenna is egregiously mistuned, causing the elements to
>couple to the feedline or tower, which then reradiate? {Again, you just
>have a note that you verify SWR before applying full power, or your rig
>automatically folds back with terrible mismatch, or, you just figure out
>that if this does happen, it actually won't exceed the MPE, regardless.)
>
>This all probably seems a bit overblown, but even professionals screw
>up. I am aware of several instances where a high power radar (low average
>power, but high peak power) with a high gain antenna was fired up, pointed
>up into the sky or out over a cliff, so the main beam wasn't pointed at
>anyone. However, it turns out that because of various near field effects,
>it was sort of dicey whether MPE was exceeded for people standing on the
>ground near the system. Just because you're not in the main beam of a 50
>dBi antenna doesn't mean the power is zero everywhere else.
>
>Jim, W6RMK
>
>
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