In article ,
Ian White GM3SEK wrote:

The version of the "non-standard J-pole" which Cebik models is fed at
bottom center, in a dimensionally-symmetrical way. His current plot
seems to show equal currents at this feedpoint, and this would seem to
make the use of a choke or balun on the feedline somewhat less
significant than with a standard J-pole.

An antenna model without a feedline will *force* equal and opposite
currents at the feedpoint - it is always fed through the perfect balun!

Cebik's plot of currents in the "standard" J-pole shows unequal
currents all the way down to the bottom, and he notes this in his text
and asserts the need for a choke.

He doesn't say whether his models do or do not include a feedline.

Add an un-choked feedline to the model (another thick wire, representing
the coax shield) at either side of the feedpoint, and see where the
current goes now. If you don't use a choke on a real-life antenna,
there's nothing to stop the current going wherever it likes.

Agreed. The results are likely to be quite variable depending on the
feedline distance to the nearest ground. Seems to me that the worst
case would result from a small integral multiple of 1/2 wavelength, no?

My guess is that in most simple J-pole installations, the feedline
radiation and the resulting disturbance of the antenna's omni pattern
are probably not going to be worth worrying about too much. Nearby
buildings, trees, etc. are likely to result in larger differences
in the far-field pattern than any quirks in the antenna's own pattern.

As an alternative to using a choke on the feedline, what sort of
results might one get with a standard J-pole by using a half-wave
coaxial balun and tapping up a bit further on the elements?

An arrangement of this sort might solve two problems at once. One of
the problems I've noticed with making the usual plumber's-delight
copper J-pole for 440, is that the 50-ohm matching points are down
quite close to the bottom shorting-bar. If one makes attachment
clamps out of copper-pipe support straps (as is suggested in many of
the sets of plans I've seen), and if the J-pole is made from 1/2"
copper pipe with elbow and T fittings, it's difficult or impossible to
achieve a good match - the attachment clamps can't be slid far enough
down on the pipes to reach the match point, before they hit the elbow
fittings.

Using a coaxial 4:1 balun would allow the attachment points to be a
bit further up.

--
Dave Platt AE6EO
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