On Mar 16, 2:33*pm, Owen Duffy wrote:
Hi Roy,

Roy Lewallen wrote ystreetonline:

...

Thanks, all noted.

What I don't understand
yet is exactly why the wire stub does what it does. It sure doesn't
work like the simplified explanations imply.

Returning to my diagram a), below is an expansion of the detail at the
junction of the stub and vertical sections.

* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *| A
* * * * * * * * * * * *B * * |
* * * * ---------------------|

* * * * *--------------------|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * *C * * |
* * * * * * * * * * * * * * *| D
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|

It strikes me that if we omit the stub all together, and leave a gap in
its place, we have two unconnected resonant elements, the top half wave,
and the bottom quarter wave with a driving source. The two elements are
field coupled to some extent, and currents will setup in each section out
of phase. NEC models support this, and I think they are correct in doing
so.

Returning now to a) with the stub connected and very close to resonance,
and with reference to the diagram above, for A, B, C and D very close to
the corners, I(A)=I(B) and I(C)=I(D).

If the desired outcome of using the stub is that the upper and lower
sections are in phase, then I(A)~=I(D). That implies common mode current
in the stub, so to cause I(A)~=I(D), the stub must have common mode
current (equal to (I(A)+I(D))/2 per conductor).

If that is true, then reduction of the physical stub to a pure
differential mode TL element is discarding part of what makes it "work".
That implies that replacement of the stub with a two terminal equivalent
impedance, eg by insertion of a load in an NEC segment, or insertion of
one port of a TL network in an NEC segment is an inadequate model.

Am I on the wrong track here?

Owen

I'm sorry...perhaps I don't understand your notation. Don't you
expect that the current at A will be (rather roughly) out of phase
with the current at D? If I think about a collinear with three half-
wave elements end to end, and drive the center of the center element,
if it's going to act like I want, I'll have high current near the
middle of each element, and those three will be in-phase. Because of
the mutual impedances among the elements, things get a bit funny at
the ends. I suppose there is a fairly large voltage across the gap
between adjacent elements, and therefore there will be moderately high
current near those ends to account for the capacitive current in the
air between them. That's what I'm seeing in the EZNEC model I just
hacked, and it's as I'd expect. The currents near the ends of the
central element are considerably higher than the currents near the
open ends of the outer elements.

(Now to spend a few minutes playing with changing the length of the
stubs through resonance...)

Cheers,
Tom