Hi Roy,

Roy Lewallen wrote in
treetonline:

....

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