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April 5th 05, 04:50 AM

wrote:
. . .
I have seen computor results
that offer 50 db F/B based on NEC, Can I trust gain if F/B cannot
be trusted?

Absolutely! While you might get some very deep nulls at some particular
points in space, and fairly deep nulls in some particular
azimuth/elevation angle combinations, they're not likely to be exactly
as deep or in the directions the program reports. Gain, on the other
hand, can be strikingly accurate in many cases.

Put together any model you want with an extreme F/B ratio. Then fiddle
the model just slightly -- change the frequency, element length or
diameter, etc. Look at how much the gain changes, and how much the F/B
changes. Modify it more, and look again.

You'll see that the F/B is *much* more critical than gain. You can goof
up the model -- or real antenna -- a lot more without any appreciable
change in gain than you can before seeing major changes in F/B.

The reason is simple. To get a deep null and therefore good F/B ratio,
you have to add the fields from all parts of the antenna together to get
zero within a tiny, tiny fraction of a percent. If any one of the fields
changes just a tiny amount, they no longer sum precisely to zero. But
small change like that won't noticeably affect the gain. No model is
good enough to precisely predict extremely deep nulls -- there's always
too much difference between the model and reality.

I don't recall what Reg recently said, but I've gotten 50 dB and greater
F/B ratios from an array by adjusting the phasing network while
listening to a receiver placed in the null direction. But the null is
that deep only in that direction, at that height above ground. It's also
noticeably shallower a little ways away even in the same direction,
because I've compensated for re-radiation from nearby objects, too. Even
coax shield leakage becomes a very noticeable factor. So while I can
tweak an array to get a very deep null, there's no way I can expect that
to hold when anything changes, even just a little. I'd even expect it to
change from day to day as the ground moisture changes and the sap rises
in the trees.

Obviously 50 db is hard to get but is it beyond the realms
of possibility?

For what, one particular azimuth/elevation combination at one single
frequency? You might be able to do it. But it would be only of academic
interest at best.

. . .

Roy Lewallen, W7EL

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