wrote:
The MFJ-962C "1.5kW" Versa Tuner III description claims to contain a
4:1 current balun. I'm using that "balanced" output on my HF
almost-doublet (slightly unequal leg lengths) with fairly decent
results.

The balun only has a single core. From what I understand, these "4:1
current baluns" aren't.

Dan,

That is correct. It is impossible to make a single core (single hole)
transmission line balun of any ratio other than 1:1. The MFJ balun, if
you actually test it, adds terrible imbalance to the system. I know
because I actually bought and measured one.

The idea for that balun came from Jerry Sevik's book about baluns and
un-uns....but unfortunately the suggestion is wrong.

Trask claims to have a transmission line balun on a single core, but he
actually has a simple isolation transformer. Isolation transformer
designs are old as dirt. You'll see in my articles on my web page and
things I wrote for ON4UN's low band DXing book that I used isolation
transformers for many years on low frequency receiving antennas, as
have many other people. The problem using them for transmitting is
loss, possible core saturation, and core heating. The flux density in
the core is very high under any load condition. They also go out of
balance badly at higher frequencies.

Bottom line is the single core 4:1 current balun used by MFJ is very
poor for balance, and subjects the core to unnecessary flux density.
I'd get rid of it for those reasons.

73 Tom

Tom is correct in every respect, though he doesn't need me to confirm
this. It is fairly easy to show that the Trask transformer is
electrically equivalent to the popular trifilar-wound 4:1 voltage balun
when wound on a single toroidal core. As such, it has no output (I
wound and measured one) into a fully unbalanced load, and of course it
has no choking action at all.

I do not know about binocular cores. It would seem the transformer
works somewhat into an unbalanced load when built with these, maybe due
to imperfect flux coupling between the two holes? I haven't measured
one. And neither has Trask himself. Until he produces a true transfer
function plot into a balanced and fully unbalanced load and a choking
impedance plot we are left to guess. To use return loss plots to infer
correct operation of a two-port network is, um, unusual.

73,
Glenn AC7ZN

On 25 Apr 2006 02:44:09 -0700, wrote:


That is correct. It is impossible to make a single core (single hole)
transmission line balun of any ratio other than 1:1. The MFJ balun, if
you actually test it, adds terrible imbalance to the system. I know
because I actually bought and measured one.

The idea for that balun came from Jerry Sevik's book about baluns and
un-uns....but unfortunately the suggestion is wrong.

Trask claims to have a transmission line balun on a single core, but he
actually has a simple isolation transformer. Isolation transformer
designs are old as dirt. You'll see in my articles on my web page and
things I wrote for ON4UN's low band DXing book that I used isolation
transformers for many years on low frequency receiving antennas, as
have many other people. The problem using them for transmitting is
loss, possible core saturation, and core heating. The flux density in
the core is very high under any load condition. They also go out of
balance badly at higher frequencies.

Bottom line is the single core 4:1 current balun used by MFJ is very
poor for balance, and subjects the core to unnecessary flux density.
I'd get rid of it for those reasons.

73 Tom

Tom and others,

I'm wondering if anyone wants to comment on Andrew Roos, ZS1AN article
in the Sept/Oct 2005 QEX issue titled "A Better Antenna-Tuner Balun"?
Andrew placed a 1:1 choke balun in tandem with a 4:1 voltage balun and
claims an improvement over the more conventional methods.

The article is available for ARRL members at:
http://www.arrl.org/qex/2005/qx9roos.pdf

73,
Danny, K6MHE




email: k6mheatarrldotnet
http://www.k6mhe.com/

I appreciate Danny's question as it allowed me to pull out that
article, which I had marked for more careful study. I'm not a
transformer expert but two things about the article strike me:

1. Andrew seems to have the transformers connected in the wrong order.
If he wants the 1:1 current balun to operate at a lower impedance it
should be on the 50 ohm side of the voltage transformer, I should
think.

2. Andrew fails to compare his scheme with its most obvious
competitor, the 4:1 two-core Guanella current balun. This would be an
interesting comparison as the Guanella can use smaller cores (did you
see the size of the voltage balun in the picture? Pretty big compared
to the current balun), but the windings operate with 100 Ohms
impedance at each end instead of 50.

But Andrew's scheme ought to basically work, and better than any single
core scheme. I'll bet the experts on this list could help further.

73,
Glenn AC7ZN