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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|