"W5DXP" wrote in message
...
On Saturday, February 15, 2014 3:27:15 PM UTC-6, Sal wrote:
As built, both the pattern
and the impedance already seem to be what I want.

And it already has a counterpoise. The counterpoise is where the current
flowing on the inside of the coax shield goes when it leaves the coax. The
J-pole has a radiation pattern of a 1/2WL vertical, i.e. the lower 1/4WL
doesn't radiate (much) and is part of the matching section and counterpoise.
Quite often, the outside of the coax braid is also part of the counterpoise.
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73, Cecil, w5dxp.com

Thanks, that's what I thought, too. The nature of current flow on the
outside of the coax is the reason so many construction articles mention
winding a few turns of the coax into a choke, not far* from the antenna. I
don't always do it but I probably should.

* But what distance is "not far"? A quarter-wave comes to mind, so a
measurement is called for. Hm-m-m ... is the propagation velocity on the
outside of the coax the same as the inside? My head spins just a little.

"Sal"

On Saturday, February 15, 2014 6:59:25 PM UTC-6, Sal wrote:
* But what distance is "not far"? A quarter-wave comes to mind, so a
measurement is called for. Hm-m-m ... is the propagation velocity on the
outside of the coax the same as the inside?

Common-mode chokes work best at the high current points so "not far from the feedpoint" of a 50 ohm antenna. Two chokes, one at the antenna and one 1/4WL down the feedline, work well. The velocity factor of a common-mode signal on the outside braid of the coax is fairly close to 1.0 only slowed down by the outside insulation layer.
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73, Cecil, w5dxp.com

"W5DXP" wrote in message
...
On Saturday, February 15, 2014 6:59:25 PM UTC-6, Sal wrote:
* But what distance is "not far"? A quarter-wave comes to mind, so a
measurement is called for. Hm-m-m ... is the propagation velocity on the
outside of the coax the same as the inside?

Common-mode chokes work best at the high current points so "not far from the
feedpoint" of a 50 ohm antenna. Two chokes, one at the antenna and one 1/4WL
down the feedline, work well. The velocity factor of a common-mode signal on
the outside braid of the coax is fairly close to 1.0 only slowed down by the
outside insulation layer.
--
73, Cecil, w5dxp.com

Thankew!

"Sal"