Dave Platt 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.

An antenna model without a feedline will *force* equal and opposite
currents at the feedpoint - it is always fed through the perfect balun!

Add an un-choked feedline to the model (another thick wire, representing
the coax shield) at either side of the feedpoint, and see where the
current goes now. If you don't use a choke on a real-life antenna,
there's nothing to stop the current going wherever it likes.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek