The Texas edu calculator is easy to use and acceptable. I use it to set my
limits. Actually the only band where I have to stay under the legal limit
is 2-meters with 380 Watts average into the antenna.
I ran each antenna through it for the legal limit key down and set my limits
by that. In actuality in anything except FM I gain at least a factor of 2
with SSB being about 5 as I recall.
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com
>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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>>
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