On Wed, 19 Feb 2014 21:16:19 -0800, "Sal" salmonella@food
poisoning.org wrote:

"Jeff Liebermann" wrote in message
.. .
... The length of the wire between the coax
connector center conductor and the driven element is an inductor. In
order to tune out this inductance, one needs a series capacitor, with
the inductor and capacitor tuned to the operating frequency.

Yes.
I've been making copper pipe and 2-wire transmission line J-poles for almost
20 years. With pipe, I usually fasten the two feed points with clamps and
slide the connections up and down. I'll get a VSWR low-point in-band but
early-on, I discovered that the best VSWR was often about 1.7:1. I had read
about (but never built) a gamma match, so I'd heard about the series cap to
tune out the inductance. I tried a series cap at the feed and it helped.

70 - 100 pF seems to be about right at 2m and I can often get a 1:1 reading
somewhere in the band.

I'm not sure of the frequency of your J-pole. The inductance of about
6 cm of #12AWG solid wire is about 0.05 uH.
http://www.consultrsr.com/resources/eis/induct5.htm
To resonate at 146Mhz, that would be about 24 pF. 50 Mhz would be
about 200 pf.

Does such a 0.3 dB improvement matter? That's not
my call. When I'm essentially playing with the technology, I can take more
time than if I'm working, like to a deadline or a budget.

0.3dB is about 6.7% loss. Probably not important or toss a coin?
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558