Owen Duffy wrote:
. . .
Understood... but, I think after our discussion on this, NEC is not up to
the task, it may take a more advanced EM field modelling tool.

I don't agree with this.

My suspicion is that NEC's shortfall is that a TL element does not
properly represent the coaxial stub and its interaction with the other
elements near resonance, though well away from resonance, it is possible
that it may be quite ok. King raises the issues of diameter ratios, and
the difference with whether the stub is inboard or outboard of the o/c
end... but it is not resolved quantitatively.

I believe that NEC can do a fine job of modeling any of the variations
we've been discussing. But like all modeling systems, it has to be used
properly -- the transmission line object isn't an adequate model for
either a coaxial structure or an open wire stub, if either is carrying
any common mode current. And in these antennas it is, so you can't
insist on using nothing more than a transmission line object and then
bemoaning that the result isn't correct. The wire stub variation can be
correctly modeled as wires. The coaxial structure can be correctly
modeled as a combination of a wire and transmission line object. In
either case I have high confidence that carefully and accurately
measured results will agree closely with NEC predictions.

Now W5GI does introduce his antenna with the statement "A multi-band wire
antenna that performs exceptionally well even though it confounds antenna
modeling software".

I know that is almost always a harbinger of bunk, the proverbial "Danger
Will Robinson...", but in fairness, it does appear that one modelling
package, NEC, cannot adequately model the coaxial arrangement near
resonance, though in his antenna, the coax section would be resonant
around 12MHz and King suggests it ought to be much shorter (resonant well
above 14MHz).

It doesn't appear this way to me at all. What has led you to the
conclusion that it isn't possible to accurately model it with NEC?
Again, it's certainly impossible if you use only a transmission line
object to represent a structure which has common mode current. There are
many ways to build a model which doesn't accurately represent the
antenna being modeled. But just because it's possible to make a bad
model doesn't mean it's impossible to make a good one.

What is the evidence that results from a properly designed NEC model
disagree with careful measurements of pattern, current, or impedance of
an actual antenna of these types? You've noted that the W5GI antenna
impedance isn't consistent with a correctly phased collinear. I'd be
surprised if the impedance isn't close to what a correct NEC model
predicts -- or that the phases of the currents aren't also what NEC
predicts.

That is not to say there aren't other BS warnings in the W5GI explanation
of operation, or claims of performance.

Thanks for your comments, I find this an interesting subject.

Me too, and thanks for bringing it up. I'd never taken a really close
look at this class of antenna before, and the results have been interesting.

Roy Lewallen, W7EL