[Antennaware] Modeling questions

[Gary Breed]

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