Many hams will recommend you try a home-made balun, multiple turns of coax
cable wound neatly around a cylindrical form. Exact requirements and
effectiveness may vary with frequency and the antenna as installed.

I saw an interesting talk at a local ham-club, at which the presenter
gave an explanation for one reason that the "effectiveness may vary"
with these sorts of choke baluns.

The common-mode impedance created by many such chokes is primarily
inductive (below the choke's self-resonant frequency, at least).

The impedance of the unwanted current path (e.g. from the antenna
feedpoint, back along the outside of the feedline, to the transceiver
chassis or to the point at which the coax is grounded) will depend on
the frequency and the length of the coax. It'll have a resistive
component (from loss resistance and from radiation resistance) and
will usually have a reactive component as well... either inductive or
capacitive.

If the feedline-path reactance is inductive or near zero, all is
well... the choke balun's inductive reactance will (if sufficiently
high) block most current flow along this path.

On the other hand, if the feedline-path reactance is capacitive, and
happens to be close in absolute value to the inductive reactance of
the choke... then you've got a series-resonant circuit. The two
reactances will largely cancel, the choke will "vanish", and you can
actually have more current flow back along the feedline than you would
without the choke.

If you change the length of the feedline, the choke's performance can
get better, or worse.

His prescription: if you want choking that's going to be effective at
a wide range of frequencies and won't be sensitive to the feedline
length, you need to use a choke which will introduce a significant
amount of resistive loss into the choked path (but not, of course,
into the differential path that feeds the antenna). The usual
solution is a ferrite.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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