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Fw: [Antennaware] Modeling questions

To: <antennaware@contesting.com>
Subject: Fw: [Antennaware] Modeling questions
From: k9ay at k9ay.com (K9AY)
Date: Tue Apr 29 17:35:52 2003
> Pete,
>
> Not sure there is a real answer to #1 and #2 -- I would suggest making a
> guess, then varying it +/- 50 percent and see if there is significant
> variation. At least you will know whether inserting a "ground loss
resistor"
> has an effect. With a single rod, ground loss will be highly variable
> anyway. And then there's the matter of how the "MININEC ground" assumes
> perfect ground for impedance calculations.
>
> My single experience with #3 had mixed results -- while constructing and
> testing my 40M beam, I attempted to calculate the conductivity so I could
> tweak the beam without more than two crank up/down iterations. I measured
> the driven element as a dipole at various heights and changed conductivity
> in EZNEC until the modeled element was about the same. This conductivity
was
> lower than the published FCC figures for the area, which was what I was
> expecting to see.
>
> Using this value in the model as a basis, I evaluated beam tuning in two
> ways -- pointing up with the reflector about 12 feet off the ground, and
at
> 30 feet in horizontal position. I chose to use the 30-foot settings as the
> first iteration and cranked up the beam to 75 ft.
>
> I did not choose wisely.
>
> After seeing how far the tuning was off (a long way), I then made made a
> proportional change back at 30 feet -- and ended up very close to the
tuning
> I had obtained with the beam pointed upwards. In this case, the model
> underestimated the frequency shift between 30 ft and 75 ft. Even so, it
only
> took one more up-and-down trial to get exactly the tuning I wanted. (See
the
> 2001 CQWW CW single band 40M results...)
>
> While the conductivity may have been a good estimate, the use of that
number
> did not help much -- I think there were too many other variables involved,
> including sloping ground, an adjacent garage, and trees. The dominant
factor
> was "effective height" rather than ground conductivity.
>
> My ramblings for today..
>
> 73, Gary
> K9AY
>
>
>
> > 1.  What is a reasonable value to assign as the loss resistance at the
> base
> > of a tower connected to real/MiniNEC ground?  The tower in question is
> Rohn
> > 25 with 3 x 8' ground rods, one at each leg, attached to the tower with
#4
> > copper wire and well-maintained connections.
> >
> > 2.  Similarly, what is a reasonable load R to assign to the ground at
the
> > outer ends of guy wires, where they are grounded through a single 8-foot
> rod?
> >
> > 3.  Has anyone had any experience with the method of determining ground
> > conductivity that was suggested, I believe, by John Belrose?  He
described
> > suspending a dipole low above ground, measuring the feedpoint impedance
> and
> > resonant frequency, and then tweaking the ground parameters in NEC-2 to
> > obtain the closest match.  If so, what frequency did you use and how low
> > did the antenna need to be to achieve the needed sensitivity to ground
> > parameters?
> >
> > 73, Pete N4ZR
>
>

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