We have found, in putting up an 8 element wire beam for 20M, that the
ground is not inconsequential in the tuning.
The thing is up at 85 feet (for reasons won't get into at the moment).
But it was necessary to measure out a reference dipole AT HEIGHT (85')
to allow us to cut the parasitic elements with that correction in
place.
Between ground effects, end insulator end-effect on the wires, and
you-change-it-when-you-put-a-center-insulator-in-it-to-measure-it,
that was a dicey exercise. A carefully designed large array can
definitely be detuned out of desired performance by these three
issues.
The bolt it and throw it up crowd, especially on "short" towers, is
definitely working with approximate tunings.
Guy.
----- Original Message -----
From: "Gary Breed" <gary@highfrequencyelectronics.com>
To: <antennaware@contesting.com>; "Pete Smith" <n4zr@contesting.com>
Sent: Tuesday, April 29, 2003 5:35 PM
Subject: Re: [Antennaware] Modeling questions
> 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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