Pete, Try this with EZNEC, using the figures I gave you, and using the
mininec ground, run the ground quality toward poor and watch the gain
go up. Use a ground constant of .001/3 and you should get a gain of
13.94, which is ludicrous.
7.4 feet is outside of your .05 lambda bogey, BTW, but I would bet
that this is digging into some quirk of mininec vs. ground.
Running the ground toward conductive causes the opposite effect. With
a very high quality ground, the model looks more like the high
accuracy ground.
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?
----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: <olinger@bellsouth.net>
Sent: Thursday, June 06, 2002 5:52 PM
Subject: Re: [Towertalk] NVIS -- modeling
>
>
> Hi guy -- More on the modeling. I don't know what is going on, but
with
> EZNEC/NEC-2, using your dimensions and the miniNEC ground produces
results
> like yours -- 10.43 dBi straight up. Using Sommerfeld-Norton ground
> produces a much different pattern, with the maxima at 41 degrees
above the
> horizon, and at only 5.99 dBi. Using miniNEC ground the shape of
the
> elevation pattern appears focused straight up, while with S-N ground
the
> pattern looks much more like the traditional low dipole as it begins
to
> move toward the standard bow-tie pattern.
>
> I tried deleting the reflector from the model using MiniNEC ground
and,
> sure enough, the pattern looks more like that using the S-N
> ground. Deleting the reflector from the S-N ground model has very
little
> effect.
>
> So it appears that the models (or at least the ground model used)
makes a
> big difference.
>
> 73, Pete N4ZR
>
>
>
>
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