Richard Clark wrote:
On Tue, 28 Mar 2006 12:59:52 -0500, John Popelish
wrote:

Air core? It is a ferrite core transformer with two one turn

Hi John,

If there's a transformer in the sense of windings; then it is an air
core, the ferrite is wholly transparent to the transverse currents.

I said that. It is a common mode transformer.

You could remove the ferrite and it wouldn't make a bit of difference
in that sense of transforming.

The short length of the two conductors passing through the ferrite is
certainly a poorer transformer (less mutual inductance between them)
if you remove the cores.

this current mismatch would cause the transformer to
produce more or less voltage across the windings

In fact, nothing of that sort happens - at least not by your
description. The ferrite is simply bulk resistance inserted into the
common mode path.

Make that, "impedance (mostly inductive, if the ferrite is well suited
to the frequency)" and I agree.

That is why common mode current is suppressed. The
same thing occurs in the coiled transmission line choke, but the
resistance is replaced by reactance. Again, common mode current is
snubbed by encountering this too.

I agree with this, except that the purpose of the ferrite is to
increase the common mode inductance of the section of coax passing
through it, not add resistance. Some resistance is inevitable,
because no ferrite is lossless, but the intention is for inductance.

The transformer property is in the isolation of the balanced circuit
from the unbalanced circuit through this resistive characteristic.

Try transmitting through such a resistance and you are going to lose a
lot of your power.

You are missing one path. The two from the source in the form of the
inner shield of the coax, and the center conductor, and the one from
the load in the form of the outer shield of the coax (same shield, but
isolated circuits).

I can't parse this. There are two metal conductors entering the
choke, and two exiting it. All currents pass through those 4 conductors.

Further, there is no flux linkage of the two
conductors coming from the source. Their magnetic lines never break
the cores,

I think you mean by this that a normal unbalanced signal in a coax has
no magnetic field external to the shield. It is all between the
center conductor and the shield. And I agree that this is what you
are trying to accomplish by adding this two conductor choke between
the coax and the balanced antenna. Without it, there would be some
magnetic field from a net (uncanceled) current and voltage on the
outside of the shield that would cause the coax to radiate. And the
voltages and currents fed to the balanced antenna would not be equal
and opposite (balanced) but somewhat unbalanced. There would also be
non equal currents in the center conductor and shield. I think we
agree on all that, but have a different picture of how a choke balun
corrects these problems.

whereas the common mode current does break the core which
thus inserts the resistance of the ferrite.

The common mode current causes flux in the core, and the conductors
passing through that flux produce a voltage proportional to the rate
of change of that flux, just as the conductor passing through any
inductor would. The transformer aspect is that since both conductors
pass through the exact same rate of change of flux, there is the same
voltage produced at the ends sticking out of the core, and this
voltage gets algebraically added to what is already there. If the
inductance of each winding is high enough (5 to 10 times the coax
impedance) a very small common mode current is enough to produce a
large enough voltage across the ends to the two conductors to correct
most of the unbalanced to balanced coupling.

Admittedly, there is no need to get this inductance (including mutual
inductance) with the aid of ferrite around the coax. You could just
wind the coax into an air core transformer. But it would be
considerable larger than one made with a high permeability core,
though, probably lower loss.