On Wed, 26 Apr 2006 09:14:24 -0600, jimbo wrote:
[snip]
The ARRL Antenna Book has a small section on J-Pole antennas. They
show both designs, conventional shorted stub and the uncommon end fed,
open stub designs. They say that the shorted stub design should have a
4/1 balun at the feed point because a direct coax connection results
in extreme sensitivity to near by objects. And they also say the end
fed, open stub design doesn't require a balun and is much less
sensitive to near by objects, but is harder to tune. (I don't
understand that point, there is one less parameter to fiddle with.)

All of this leads me to wonder why almost all J-Pole designs one sees
are of the shorted stub version and almost none use a balun. I suspect
ease of construction and low cost. My "store bought" Arrow end fed,
open stub, J-Pole seems to verify the ARRL Antenna Book's conclusions
about sensitivity to near by objects. However, do-it-yourself
construction does appear to be much more difficult. I may give it a
try, just out of curiosity.

[snip]

A j-pole (open or closed stub) will perform better with a common mode
choke at the feed point. Even better, use two chokes. One at the feed
point and the other 1/4-wave down the line.

This will reduce the higher angle radiation that results from the feed
line being part of the antenna.

Using a choke on ANY antenna fed with coax is just good engineering
practice. It can't hurt - only help. A few turns of coax costs very
little.

73,
Danny, K6MHE


In my many years I have come to a conclusion that one
useless man is a shame, two is a law firm, and three
or more is a congress. - John Adams

email: k6mheatarrldotnet
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