On Mar 17, 1:31*am, Owen Duffy wrote:
K7ITM wrote :

...

Yes--and then if they were exactly equal, would that not imply only
transmission line current on the stub? *Obviously, they are exactly

Thinking some more about it, my current thinking is that my analysis was
flawed. I was using the standing wave currents, when I should be using
the travelling wave components.

I suspect that when NEC models the conductor arrangement at my fig a), it
correctly accounts for propagation delay and the phase relationships
compute correctly.

If we replace the stub with a TL element, I suspect that NEC reduces the
TL to a two port network and loads a segment of the vertical with an
equivalent steady state impedance of the s/c stub network. If that is
done, the reduction to a lumped load means that there is zero delay to
travelling waves, and the computed currents (amplitude and phase) in the
vertical will be incorrect. This means that you cannot replace a resonant
stub with a high value of resistance, it doesn't work.

If that is the case, it suggests that NEC cannot model such phasing
schemes using TL elements.

Owen

Of course, if the TL model doesn't "know about" the antenna field
(which I believe is in fact the case), there will be no common-mode
current on it because of that field. It's pretty clear to me that the
common-mode current is very important to correctly simulating the
situations you are interested in. In fact, figure (B) of your
original posting puts the stub in a position where it does not see the
antenna field, and I would expect it to behave much differently from
the perpendicular stub of figure (A).

One of the things I did in my simulation playing last night was to
delete the stubs, leaving just the three 1/2 wave elements end-to-end
with a bit of gap between them. (0.01m gap between 0.5m elements, 1mm
diameter, 11 segments each.) I'm sure you know what that pattern and
current distribution look like. Then I added sources at the centers
of the outer elements. I set all the sources to 1 amp, in-phase. The
pattern was somewhat sharper (though just marginally more gain) than
the stub-coupled case. What I didn't try, but will as I have a
chance, is to put sources at the centers of the outer elements and set
them to the values (magnitudes and phases) I see in the stub-coupled
collinear, and see how much the current distribution near the ends
looks like the stub coupled case. I suppose it will be pretty close,
and the antenna pattern will look very similar to the stub coupled
pattern.

Thanks for bringing this subject up. I'm learning something from it.

Cheers,
Tom