Joe,
I model short and full size verticals, build them and they work as modeled.
For the top loading inductance I leave about 10% of the loading inductance
at the base to account for modeling errors and to be able to tweak it from
the ground. So far no problems.
For modeling ground losses I place four resonant radials a couple inches
above the ground. Then add a base series resistor load to represent ground
losses. I often use 10 to 20 ohms based on ground measurements I have
made. Rudy's data can be used for determining the value of the base
resistance.
I also have a work around for NEC-2 to model ground losses that involves
placing resistive loads periodically along the four radials.
So far I assume that NEC-2 accurately (enough) models the far-field ground.
My feedback on this has been the performance enhancement I notice when I use
The Great Salt Lake of Utah as a ground. With it extending for 100
wavelengths at 1.8 MHz to the East, NEC-2 claims 10 dB over the same full
size vertical at my house with its 'average' ground. I have run the balloon
vertical at both locatons several times but not at the same time for A-B
comparisons. The proof of the salt lake performance enhancement is how well
it contests with 5 watts.
Dave WX7G
On Mon, Mar 8, 2010 at 11:59 AM, Joe Giacobello <k2xx@swva.net> wrote:
> I understand the reservations regarding modeling ground mounted
> verticals. However, I've had very satisfying results in modeling
> quads. In one case I modeled and built a six band quad with four
> elements on 10-20M and seven on 6M. I had similar results with a two
> element quad for 30 and 40M. My criterion for success was that each
> antenna required little or no pruning to get the resonant frequencies
> somewhere within the bands, if not subbands, of interest. I did not even
> attempt to verify gain or F/B. I had equally good results when modeling
> a 40M vertical with four elevated radials, which was obviously not a
> very demanding task. That six band quad had something approaching 1200
> feet of wire in it.
>
> When Rudy, N6LF, published his experimental data for radial number,
> length, etc. for 40 and 160M ground mounted verticals on his website, I
> tried to develop a correction factor that might allow EZnec to conform
> to his data, but I could come up with no useful pattern. It just didn't
> make sense and I threw in the towel.
>
> When I modeled my own dual band, ground mounted vertical for 80 and
> 160M, I used some of the tips that K2AV had suggested in various posts.
> If I recollect correctly, the model got the reactance close to right but
> the radiation resistance was off (high) by about 50%. Nevertheless, the
> model was useful for providing direction on the effect of top hat
> radials, perimeter wires, etc. The resonant frequency of the unmatched
> vertical was 2.3 MHz and the model predicted 2.15.
>
> I'm no expert in either antennas or modeling, but those are some of my
> experiences. Given the relative ease and reliability of modeling these
> days, I wouldn't consider building antenna without first modeling it.
>
> 73, Joe
> K2XX
>
> K9AY wrote:
> >>> Modeling of antennas? Design, build, it works as modeled.
> >>> WX7G
> >>>
> >> I have all the scars and memories of wasted time that would prove
> >> otherwise.
> >> K2AV
> >>
> >
> >
> > When the finished product works exactly as modeled, it is ALWAYS a
> > combination of:
> >
> > a) The model's ability to represent reality.
> > b) Consistent, predictable behavior of the construction medium.
> > c) The designer's ability to know when his/her design satisfies (a) and
> (b),
> > so he/she can stop tweaking the model and get it built.
> >
> > This applies to antennas, hypersonic aircraft, or the latest
> > multi-band/multi-mode SDR transceiver chip for your smartphone. As Dave
> > notes, many things are accurately built based on modeling. But as Guy
> says,
> > there are also many cases where (a) or (b) is insufficient.
> >
> > 73, Gary
> > K9AY
> >
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> >
> >
> >
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