On Fri, 27 Oct 2006 13:31:57 +0100, Ian White GM3SEK
wrote:

....
A couple of days ago, Bill re-quoted the WA2SRQ measurements, which are
the same ones we've been discussing for the past week (seems like more
:-)
http://www.bcdxc.org/balun_information.htm#Ed,%20WA2SRQ

However, it's very interesting to read the whole of that web page, which
is a much longer discussion involving several other designers and users
of feedline chokes (aka choke baluns). In that discussion, there was a
largely unspoken agreement that, to merit being called "effective", a
choke should have an impedance of at least 10 times the cable Zo, ie at
least 500 ohms.

If 500 ohms is all you need, a coiled-cable choke of either the
"bunched" or the "solenoid" type certainly can cover at least two
amateur bands an octave apart in frequency... but a wide range of
ferrite chokes can do the same, and these have the advantage of being
much more broadband so they need no tuning.


Ian,

I agree that this criteria is oft cited, but it bears examination.

It seems to derive from a bench test of a balun, where the balun
shunts the balanced load (on one or both legs) by some impedance,
often assumed to be purely inductive reactance (though that is not
true from some baluns, eg the common W2DU style), and that if that
impedance is 10 or more times the balanced load impedance, then the
impact of the shunt reactance is negligible, and the common mode
current caused by a single leg shunt is negligible.

This might be a reasonable criteria for a balun in a bench situation
or equipment room situation (eg between instruments or boxes with a
mix of balanced and unbalanced interfaces), it should lead to low
insertion VSWR which might be important if one was VSWR focused or
obsessed.

I suggest that such a criteria is not complete enough in itself to
predict the impact of the balun on common mode current or balun loss
in an antenna + feedline + transmitter + ground scenario.

NEC modelling of some scenarios suggests to me that effect of common
mode chokes at different frequencies depends not only on their
impedance, but also on their location, and that sometimes more than
one choke may be more effective than a single larger choke.

Those models also reveal the standing wave nature of the common mode
feedline current, and the futility of taking a current probe
measurement at a single location to infer any more than the current at
that specific location (if that was important).

Owen
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