Owen Duffy wrote:
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.


That was very much what I said, farther down the message from which you
quote.

Impedance isn't everything, but it does show the important difference
between "good" chokes that are capable of making a large change, and
"poor" chokes that will always have a much smaller effect.

The next thing is a different point, which can limit the effectiveness
of even the best of chokes.

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).

Common-mode current is well known to be "standing wave" in nature, in
the sense that it has maxima and minima alternating along the line,
separated by an electrical quarter-wavelength.

If you disrupt this pattern by inserting a common-mode choke at a
current maximum, you're going to force a current minimum at that point.
(That can only happen if the choke has a sufficiently high impedance,
but let's assume it has.) RF currents on the entire antenna-feedline
system will then rearrange themselves to take this new factor into
account.

But something must have caused this tendency to have significant
common-mode currents in the first place - most commonly an asymmetrical
layout of the antenna and/or the feedline, eg when the feedline to a
dipole runs parallel to the antenna underneath one side. A common-mode
choke can treat the symptom (the common-mode current) but it cannot
remove the root cause (the layout). In such cases, inserting a choke
will cause a new common-mode current maximum to pop up, a
quarter-wavelength away.

Now if you insert another choke at this new current maximum, you have
made it extremely difficult for the line to support any common-mode
current between those two chokes - but yet another new current maximum
will pop up, a further quarter-wave away. So on you go, rather like
trying to squeeze down a long balloon...

However, you are actually making progress. Every correctly placed choke
makes it more difficult for common-mode current to exist on the line,
and takes you farther away from the local field of the antenna.

The name of the game is to reduce the common-mode current along the
whole of the feedline, as much as you can, but above all to reduce the
current at the location of the equipment that is being affected (eg the
transceiver). Therefore it's always important to monitor the current at
the victim equipment, and be aware that it can sometimes increase.

In practical terms, the two most important places to try a common-mode
choke a

* At the end of the coax, where it breaks out to feed the antenna, or
into parallel line, because this is where common-mode current can be
launched onto the outside of the coax by a direct hard-wired connection.

* At the victim equipment in the shack, or at some "gateway" point like
the ATU.

Because of the "pop-up" nature of the current maxima, there are always
exceptions for certain layouts and feeder lengths, but these are two
good practical places to start.

It can be difficult, and an RF current probe will be your best friend
and faithful tracker... but it's never futile.





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