[TowerTalk] RF Exposure Calculator

Jim Lux jimlux at earthlink.net
Mon Apr 24 21:08:51 EDT 2006

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:
>                          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.


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 

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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