Dave Platt wrote:
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.

If they did, the configuration of the feedline would become another
important variable which he couldn't fail to mention. Also the drawings
show no 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?

That's right. At the point where the coax shield connects to the
antenna, the current will divide three ways, between the antenna, the
inside of the shield, and the outside of the shield. The split will
depend on the ratio of the impedances in each of those three directions.

If the impedance for current flowing down the outside of the shield is
low (which any multiple of 1/2 wavelength grounded at the bottom will
achieve) then away the current will go - there's nothing to stop it.

A feedline choke creates a high impedance against current flow down the
outside, so the current from the inside of the shield flows almost
exclusively into the antenna.

Even if you choke a J-pole at the feedpoint, there will also be induced
currents further down the feedline because the antenna and the feedline
are usually installed in a straight line. But that doesn't override the
need to choke the feedline at the most obvious place.

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.

Most people using J-poles won't worry, that is true... but that's mostly
because J-poles are used in relatively undemanding applications where
you either hit the repeater or packet node, or you don't.

In defence of Al, if the J-pole is mounted directly on a car roof, then
there's no point in attempting to choke the feedpoint. But if it's
mounted on a mast, the mast and feedline will radiate. How much will
depend on the exact installation, and is pretty well unknown unless you
can measure the actual RF currents.


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?

That is a workable feed method, but the half-wave balun is a voltage
balun. The antenna is asymmetrical, so the balun is acting as
center-tapped voltage source which is trying to push equal currents into
the unequal impedances on either side of the feedpoint. That will never
quite succeed, so there will always be some out-of-balance current left
over.

Like all voltage baluns, the half-wave coax type doesn't do anything
directly to *prevent* the out-of-balance current from flowing away on
the outside of the feedline. On the contrary, there is a hard-wired
connection that will *allow* such currents to flow.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek