On Sun, 12 Oct 2014 18:13:39 -0000, wrote:

There are lots of plans out there for J-poles made of pipe where one
of the legs is extended at the bottom to provide the mounting, typically
by bolting it to a mast. The assumption is there is no current in that
bottom leg.

If you model that you find that there really is current in that bottom
leg and it can REALLY screw up the antenna characteristics.

I don't like J-Poles. Various reasons, some of what are
non-technical. I'll spare you the details.

Our local radio club had a meeting where everyone was suppose to build
a twinlead J-Pole or a Slim Jim and have it tested by the experts and
their MFJ antenna analyzers. It wasn't too difficult for me to
predict that things were not going to work according to the science
fiction found in the collection of construction articles, so I elected
to just attend and watch. I arrived late and found about 12 J-Poles
in various stages of construction and testing in groups of 2 to 4
members. Many J-Poles were not even close to tuning onto the target
frequencies. All of them changed tuning when the antenna was moved.
Measurements were stable unless something moved (including people),
where everything changed. Someone had the bright idea of hanging the
antennas from the metal framework supporting the acoustic tile
ceiling, thus adding the framework to the antenna. There were no
baluns in sight, so when I recommended adding some additional coax
cable just to see what happens, the tuning again changed radically.
Running my hand up and down the coax cable did much the same thing,
demonstrating that a balun was probably a good idea. Minimum VSWR
improved when I convinced two builders to add series trimmer caps to
tune out the inductance of the feed tap wire, but usable bandwidth
decreased. In the past, someone would usually bring a copper pipe
J-pole with the coax feed and ground reversed, and wonder why it
wouldn't work. Not this time.

I tried to drag out my laptop and show some interested builders an
NEC2 model of their J-Pole, but couldn't figure out how to deal with
the velocity factor of elements made from twinlead. The IS card in
NEC4 handles it easily, but there's no equivalent in NEC2.

Incidentally, I use an insulated PVC pipe for the "handle", large
diameter elements, a balun, and a series tuning cap (Gamma match
style), when building a J-Pole.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558