[TowerTalk] Tips for Modelling swaged antenna tubing sections

[K9MA]

A somewhat more general question:

There is a whole bunch of trapless tribanders available now, all based 
on modeling. How well do the predictions of NEC4 models for these 
antennas really agree with actual test results? Antenna testing is 
expensive, and I know at least one vendor admits to not testing at all, 
relying entirely on the model results.  I have a couple of such 
commercial antennas, and I'm not convinced they really perform as the 
model suggests. I've done some pattern measurements for one of them 
under almost exactly the same conditions for which I measured the 
pattern of my old TH7. The TH7 looked consistently better and the TH7 
patterns looked a lot like those published by HyGain, which were 
certainly based on antenna range measurements.

The tuning of yagi parasitic elements is quite critical, so probably 
requires much better model accuracy than, say, a dipole. For example, 
AFAIK all these model-based tribanders have isolated parasitic elements, 
because the effect of the boom can't be modeled easily. The parasitic 
elements of the old trapped tribanders could be connected the boom, 
because they were initially tuned empirically.

Does anyone know of any measurements made to verify model results for 
this sort of antenna? (Even on a scaled VHF/UHF model?)

73,
Scott K9MA


On 10/1/2020 7:59 AM, Steve London wrote:
> Jim Lux wrote:
>
> but does the model actually show much difference? One can get way down 
> in the weeds with this - put a tapered segment in that's 1 cm long, 
> etc. But if the wavelength is 20 meters, a 1 cm transition is 0.0005 
> wavelength. I'd worry more about numerical instability than model 
> accuracy at that point.
>
> --------------------
>
> I have been going through the same exercise with a homebrew 15 meter 
> beam. Can models like N6BV's YW be trusted for segments that are a 
> small fraction of a wavelength ? What about the Leeson 
> equations/spreadsheet for converting to an equivalent, 
> single-diameter, cylindrical element ?
>
> Some examples from my modeling:
>
> - The 1/2" wide, 0.1" thick, 3.5" long, aluminum straps that connects 
> together the parasitic element halves ?
>
> - The 3" long swaging sections ?
>
> 73,
> Steve, N2IC
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-- 
Scott  K9MA

k9ma at sdellington.us