On Tue, 28 Mar 2006 22:33:18 -0500, John Popelish
wrote:
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.

Hi John,

No, the principle of the impedance is overwhelmingly resistive, not
reactive. One core (#75) that snuggly fits over RG58, at about 10MHz,
exhibits 25 Ohms resistance to common mode current. My BalUn uses 50
such whose combination presents 1250 Ohms resistance. This value
varies over frequency, but easily finds a 1000:50 ratio over the
entire MF/HF Ham spectrum.

I also have two engineering samples boxes full of various material
types and geometries and have measured them all. Some exhibit
reactance, but are vastly overwhelmed by their intrinsic resistance.

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.

Quite the contrary. Inductance, or more properly to the application,
reactance, may come as a consequence of these cores, but that is
really just a minor component to the vastly greater resistance.

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.

That power, actually current, was forced into a common mode because of
one of several problems:
1. Massive imbalance in the radiator design,
2. Massive catastrophe with the radiator.

If you are destined to "lose a lot of power" into such resistance, you
have far greater problems you are ignoring. However, let's return to
this "loss of a lot of power." You have an antenna with a nominal 50
Ohm load resistance (presumably balanced to the common). You have a
common mode choke with 1000 Ohms resistance in parallel (to the common
of the common mode). Now, why would the power choose the path of 10
to 40 times more resistance to invest its calories in?

I will be generous and pour 1.5KW into a balanced 50 Ohm load to
develop 275V that is also across the 1000 Ohms of the BalUn. 76W into
50 beads. Now this verges on serious power for this load I will
admit, but it also presumes key-down conditions which would only be
found for RTTY or SSTV. By my bench measurements, I figure that the
core's can tolerate as much power as the bulk equivalent carbon
composition resistor - or least this is my rule of thumb. This means
about 1 or 2W per bead to offer enough heat to cause pain, but perhaps
not enough to blister. This is another benchmark from my days in the
Navy.

I once asked an ET Chief how much heat a transistor could tolerate. He
said
"If you can hold onto it and count to 10, it's not too much."

I've been blistered by TO-5 cans with just a casual brush and they
still filled their design mission.

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.

Your count is four terminals (not conductors), there are six. You
need to come to terms with this shortfall in your count as it explains
the utility and design of BalUns.

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.

That is apparent.

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.

This "rate of change" is spurious, call it frequency dependant, but
then frequency dependency is neither here nor there at this moment.
More to the matter trying to turn this BalUn into a power transformer
is doomed in this analysis as it has absolutely no impact to
correct
most of the unbalanced to balanced coupling.

73's
Richard Clark, KB7QHC