On Mon, 13 Jun 2005 18:07:13 -0000, (Dave Platt)
wrote:

In article ,
Cecil Moore wrote:

By open stub, you mean there is no loop at the bottom of the "J"?

He means the bottom of the stub is the feedpoint, like this:

+------------------------------------------------------------
|
FP
|
+-----------------

And that looks like an unbalanced feedpoint so it should result
in common-mode currents, sorta like an off center-fed dipole.

If I recall correctly, Cebik has some discussion and modelling of such
open-sleeve J-pole designs in his "Tales and Technicals" section.
Start at http://www.cebik.com/vhf/jp1.html and go down about half-way.

The version of the "non-standard J-pole" which Cebik models is fed at
bottom center, in a dimensionally-symmetrical way. His current plot
seems to show equal currents at this feedpoint, and this would seem to
make the use of a choke or balun on the feedline somewhat less
significant than with a standard J-pole.

But he says, "However, since the currents on either side of the
feedpoint are not balanced, a choke balun is mandatory to suppress
unwanted currents on the feedline."


Cebik's model would need to be modified somewhat apply to the Arrow
open-sleeve J-pole, as the Arrow is fed at the very base of the one
element rather than halfway between the two.

That's how I modeled it, although it makes very little difference in
the results.


Anybody care to model up the Arrow, with all three elements included,
with and without a separate wire representing the outside of the coax?

Done, before you asked.[g]