[TowerTalk] Tips for Modelling swaged antenna tubing sections

[jimlux]

On 10/1/20 10:39 AM, K9MA wrote:
> 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?)
> 

For high frequencies, at JPL, we use a variety of modeling tools.  If 
the measured performance differs from the model, we assume that the 
antenna was incorrectly assembled.  That is, we trust the model more 
than the measurements, particularly for "off boresight" sidelobe levels 
and such.