[Antennaware] Re: NVIS -- modeling, mininec vs nec-2 disparity

[Guy Olinger, K2AV]

Finally dug up the specifics of the disparity.

To say that Mininec is "inaccurate under .2 wavelength" because ground
loss inaccuracies can become "significant" really doesn't do the job
about when to switch to another method.

The following snippet from the Doc for AO. The methods are Mininec
methods, which the optimizing shell in AO uses as a core.


              "You should exercise caution when modeling antennas over

      ground.  AO uses ground characteristics only to determine the

      ground-reflection factor for the far-field tabulations and

      patterns.  It uses a perfect-conductivity groundplane when it

      calculates wire currents.  This implies that ground-current

      losses are not accounted for.  While these losses normally are

      negligible for horizontal antennas higher than about 0.2

      wavelength, they can be significant for low horizontals and for

      verticals fed against poor ground systems."

Or as you go past .2 wavelength, start worrying whether you are
ignoring near-field ground losses. It's not like the model falls off
the table at some point approaching ground. It DOES fall off the table
on some other issues like parallel wires closer than something like an
inch. Go past that bogey and it gets quickly strange.

Frankly, this little exercise is the first AO generated solution of
mine that had anything in it that showed any diff with NEC other than
minimal center frequency or impedance diffs.

Apparently seven feet off the ground for the reflector is
detuning/lossing up the reflector.

Whether that is a loss related issue or AO picking a length that ought
to be something else, remains to be seen. AO stuck the reflector down
there because it thought it was hooking some ideal ground
reinforcement. Seven feet not all that close. Suspect there is a
better length/height for the reflector.

It is much harder to generate iterations through EZNEC looking for a
solution than to let AO go looking for it automagically.

Nevertheless, I owe you a beer or two, Pete...

Will constrain the height of the solution and see what that reflector
is at 28 feet.

----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: <antennaware@contesting.com>
Sent: Friday, June 07, 2002 7:17 AM
Subject: [Antennaware] Re: NVIS -- modeling, mininec vs nec-2
disparity


> At 09:51 PM 6/6/02 -0400, Guy Olinger, K2AV wrote:
> >Going to post this to antennaware, with the figures to see if the
> >gatheren already know about this.
> >
> >The antenna in question...two element reflector beam pointing
straight
> >up.
> >
> >7.025 MHz, upper wire fed in center
> >
> >Upper wire = 67.5 feet long, 47.7 feet high (21 segs on eznec, 16
> >segs/hw mininec)
> >Lower wire = 68.4 feet long, 7.4 feet high
> >
> >AO (mininec) optimizes to these numbers, gets 11 dbi at 75 degrees
> >with 11.4 dbi straight up.
> >
> >Anyone know about this one and care to comment?
>
> I'm not sure how much of the context was on the reflector, so let me
just
> add a little.  The same model, with NEC-2 and Sommerfeld-Norton
ground,
> shows a VERY different result.  Instead of peak gain being at the
zenith,
> it is at 41 degrees and maximum is only 5.99 dBi.  With what EZNEC
calls
> MiniNEC ground, results from NEC-2 are close to those Guy reports
from
> AO.  Since doubling the number of segments produces very little
change in
> these results, I presume they represent good convergence.
>
> The question is, which modeling result is closer to reality?
>
>
> 73, Pete N4ZR
>
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