LAB wrote:
CECIUL MOO
How can one move the feedpoint up and down the matching stub with
insulation on the wire?

I started at the same distance of the copper tube antenna. About 1cm
apart SWR seemed to be about the same: not better, not worse. It seemed to
be the best... I've had no time in the week for other tests; I'll continue
today or tomorrow. If someone has an idea it will be appreciated.

The velocity factor of the twinlead might be as much
as 20% different from the copper tubes. If you use
the same dimensions, the resonant frequency will be
too low. Did you compensate for velocity factor?
Did you check the resonant frequency with something
like an MFJ-259B?

By changing the characteristic impedance of the
matching stub, you change the optimum tap point
on the stub so you cannot use the same dimensions
as the copper tube antenna.

Seems you need to make the adjustments using a
throw-away version of the antenna. Once you determine
the proper dimensions, you can build the final version.

I would start by shaving the insulation off both
stub wires so they can be soldered to the coax and
chopping off 10% of the length of the 1/2WL section.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com