Well, I'm pleased to report I'm not going senile - well, I don't think so!
I'd forgotten that I saw a similar problem last year on a Hexbeam model
and contacted Roy Lewallen about it. I just found the model again and
can reproduce a very similar bug:
Double click the file to open EZNEC
Click FF Plot
Azimuth plot shows Gain -3.47dBi, Front/Back 26.58dBi
Click Ground Type
Click Free Space
Click OK
Click Ground Type
Click Real
Click OK
Click FF Plot
Azimuth plot shows different results: Gain -4.04dBi, Front/Back 40.13dBi
I get the first results consistently when I've loaded the file until
such time as I select Free Space and revert to Real Ground; thereafter I
consistently get the second set of results. The model is complex and
produces a segmentation warning, but I can't understand why the results
wouldn't be repeatable.
Notice that I don't even need to do a computation run to correct the
problem - simply selecting Free Space and then immediately re-selecting
Real Ground is enough to correct it. I sent Roy the file in question but
didn't ever hear back from him.
If anyone would be interested to check whether this behaviour is
repeated on their version of EZNEC - I'm on EZNEC+ v.5.0.20 - I'd be
happy to forward the file.
Steve G3TXQ
Steve Hunt wrote:
> Jerry,
>
> My head is spinning after 30 minutes of trying to compare our figures.
> I'm sure you wont believe this, but I believe there is a bug in EZNEC.
>
> I loaded up the vertical model I was using last night - 132ft over
> Real/MININEC average ground, Aluminium Wire Loss, and re-ran the
> figures. My numbers were several dB higher than yours. I then switched
> to Perfect Ground and Zero Wire Loss to check my Average Gain figure -
> it was very low. Then I switched straight back to Aluminium Wire Loss
> and Real/MININEC ground and got a completely different set of figures -
> very close to yours.
>
> I repeated the exercise from a "cold start" (closing down EZNEC and
> restarting it) about 6 times and consistently saw the same thing - an
> erroneous set of figures at first, and then a correct set once I'd
> swapped to Perfect Ground and back. I was trying trying to bottom-out
> exactly which change caused the change in behaviour, when it stopped
> doing it and I now can't reproduce the effect.
>
> I wonder if anyone else has ever seen this? Please don't suggest it was
> "operator error" - once I'd first identified the effect I was very
> careful to check and double-check every setting.
>
> Steve G3TXQ
>
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