Oop, looks like I done hit de tar baby.

Gene Fuller wrote:

I have read your comment about the requirement for two cores for many
years, and I am still puzzled. In the ideal case the balun action is
performed by transmission line transformers, and the flux induced in the
core by the differential current is minimal. The core primarily affects
frequency response and common mode choking action.

It's true that the flux induced by the differential current is minimal
in a transmission line transformer. If, in fact, the balun is wound with
coax as I usually make them, the flux induced by the differential
current is zero because the differential field is entirely inside the
shield.

And it's true that the core affects common mode choking action. The
common mode choking action is exactly what the function of a balun is --
that's how it effects equal and opposite (purely differential mode)
currents on the two transmission line conductors. In other words, that's
how it produces balance and prevents feedline radiation.

So the flux from the common mode component of the current is the very
thing we need to look at when we evaluate baluns.

According to his books, Jerry Sevick has designed, analyzed, built, and
measured many single core 4:1 current baluns. He never mentions the
problem you allude to.

I don't know which edition you have. I have only the first and second; I
believe there was a third. In the first edition, it was clear that
Sevick didn't understand the purpose of baluns or how they accomplished
it. So I discount anything he wrote about baluns in the first edition.
But he figured it out some time between the first and second editions,
so there's some credible information in the second. A number of the 4:1
baluns he shows are indeed wound on a single core. And he dutifully
reports their frequency responses when terminated in a correct impedance
load (seldom the case in a real antenna application). But nowhere do I
see any measurement of their ability to do the primary job of a balun:
to balance the feedline currents. So there's no way of knowing whether
any or all of the many "current" baluns he constructed actually perform
the function of a current balun.

A way to qualitatively check to see if a balun is really acting like a
current balun is to connect a load across the output. Short one terminal
of the output to the "ground" terminal of the input, and measure the
impedance looking in. Then disconnect that short and short the other
output terminal to the "ground" terminal of the input and measure it
again. You can do this with an antenna analyzer or impedance bridge. A
perfect current balun will show the same, correct impedance in both
tests. A perfect voltage balun will show very different impedances.
There should be a way to get a quantitative measure of the longitudinal
(common mode) impedance from this test, but I don't have time to dig it
out just now.

I'd like to add that not long ago I encountered a claim of a 4:1 current
balun on a single core. It was indeed -- it was a conventional, as
opposed to a transmission line, transformer, although it was wound in a
way that made it resemble a transmission line transformer. And it had a
fairly decent bandwidth, although a transmission line transformer is
inherently much broader and doesn't couple differential flux into the
core. So a 4:1 current balun can be constructed on a single core. But I
don't know how a 4:1 transmission line current balun can.

What is happening here?

A whole lot of things could be happening. If an antenna is perfectly
symmetrical, a voltage balun will do the same job as a current balun.
For an explanation why, see
http://eznec.com/Amateur/Articles/Baluns.pdf. So a lot of the "current
baluns" might well be voltage baluns. I seldom see anyone actually test
the current balancing properties of baluns, and without this, there's no
telling what's been constructed.

I haven't looked carefully at Sevick's single core 4:1 "Guanella"
baluns. They might be voltage baluns, or they might be conventional
transformers -- bifilar windings don't automatically create a
transmission line transformer. I don't see any test he ran to verify
that they're functioning as current transformers or for that matter as
transmission line transformers. In a transmission line transformer, the
differential current always has a transmission line path to follow. Some
connections can split the wires carrying a differential current apart,
in which case you no longer have a transmission line transformer.

If you know of a topology which produces a transmission line 4:1 current
balun, I'd be very interested. You could use the same simple analysis
method used in the balun article to show that it really is what you think.

Mike Coslo wrote:

I'm not sure myself, Gene. I've wound single core 4:1's, I've bought
at least 2 of them. I've also been told variously they don't work, and
can't even be made.

Just what IS happening here?

You probably made 4:1 voltage baluns, which worked well enough in your
applications. Where a voltage balun is really a bad idea is when your
antenna system isn't symmetrical. In an effort to balance the voltages
on the two output terminals, it creates unbalanced currents. The
difference between them becomes a common mode feedline current. This is
also explained in the balun article.

.. . . Here Uncle Remus paused, and drew a two-pound yam out of the ashes.

"Did the fox eat the rabbit?" asked the little boy to whom the story had
been told.

"Dat's all de fur de tale goes," replied the old man. "He mout, an den
agin he moutent. Some say Judge B'ar come 'long en loosed 'im - some say
he didn't. I hear Miss Sally callin'. You better run 'long."

Roy Lewallen, W7EL