Cecil Moore wrote:
wrote:
8 turns loose wound in a 6.625" diameter bundle looks like an effective
single band balun for 80m (approx 500 ohms).

This would only be true for a resonant coax-fed 1/2WL
dipole. What about all the other possible configurations?

A ladder-line fed dipole might present an impedance of
8000 ohms to the choke. A 500 ohm choke would have
very little effect. However, a self-resonant choke
might have 40K ohms of choking impedance and it
would need to have a lot more turns than 8.
--
73, Cecil, http://www.qsl.net/w5dxp

Cecil Moore wrote:
wrote:
8 turns loose wound in a 6.625" diameter bundle looks like an effective
single band balun for 80m (approx 500 ohms).

A ladder-line fed dipole might present an impedance of
8000 ohms to the choke. A 500 ohm choke would have
very little effect. However, a self-resonant choke
might have 40K ohms of choking impedance and it
would need to have a lot more turns than 8.


There's a disconnect between these two statements: they make very
different assumptions about the level of performance that is needed.

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.

In contrast, Cecil is asking for a much higher impedance. If that's what
you really need, then nothing can beat a resonant coiled-cable choke,
which can give a resonant impedance well above 10kohms and possibly as
high as 40kohms [1]. Even the very best ferrite choke can't come
anywhere near that... but the resonant choke will always be a
single-band device and will always have to be adjusted carefully.

[1] Measurements on various kinds of chokes, at:
http://w8ji.com/Baluns/balun_test.htm (table about half-way down the
page).


The performance of a common-mode choke is always going to depend on the
exact situation in which it is used, so the measured impedance can never
be more than an indication of its *potential* performance. For example,
a 500-ohm choke may completely kill the common-mode problems at one
station (or on one band) and have almost no effect in a different
situation.

On balance, it might be better to say that 500 ohms should be considered
the *minimum* value of impedance that can offer some promise of being
useful in a wide variety of different situations.

But maybe Cecil is setting the bar too high by imagining the worst
possible case. If an antenna/feedline system has a common-mode problem
that needs an almost impossibly high value of choking impedance, then
(as I said earlier) the choke isn't really the thing we should be
looking at. It's the root cause of that problem - the antenna and/or
feedline itself - that needs attention.


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