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

For what it's worth, I think you're on exactly the right track, Owen.

Some things to ponder: does it make any significant difference if the
stub is, say, 2mm wires spaced 20mm apart or 1mm wires spaced 10mm
apart (that is, the same impedance line, but different physical size),
and does it make any significant difference if the wires are kept in a
plane that includes the antenna elements, or if they are twisted near
their attachment point so they lie in a plane perpendicular to the
antenna wire, or if they are twisted throughout their length? What if
they are coiled in a spiral out from the antenna wire, so their
shorted end lies much closer than a quarter wave from the axis of the
antenna? I don't have any answers to these questions; they just seem
like an interesting and reasonable extension of your original
question.

Cheers,
Tom