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.

Furthermore, my antenna presents both higher and lower impedances than
50 ohms depending on band.

For convenience, I'd like to have a single port that I connect my
antenna to at all times. I've remoted this tuner and I don't want to
make more complicated switching arrangements. Is it worthwhile to
rewind the "4:1 current balun" as a 1:1 choke balun?

It seems that it might be more appropriate for my application. I
should expect the core to work well for any HF transmission line
transformer I'd like to wind, as long as I don't saturate the thing,
right?

73,
Dan
N3OX

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.

I don't know of any way to make a 4:1 current balun with a single
one-hole core. I think it's possible with a two-hole (binocular) core if
done right.

Furthermore, my antenna presents both higher and lower impedances than
50 ohms depending on band.

The "4:1" transformer probably has a 4:1 impedance transformation only
when the load Z is near 200 + j0 ohms. Elsewhere, it'll have a different
transformation ratio and also will add series and/or shunt impedance.

For convenience, I'd like to have a single port that I connect my
antenna to at all times. I've remoted this tuner and I don't want to
make more complicated switching arrangements. Is it worthwhile to
rewind the "4:1 current balun" as a 1:1 choke balun?

Maybe somebody else can take a crack at this, but I think it's
impossible to say. It depends on both the differential and common mode
impedances seen at the feedline input as well as the tuner's matching
range and efficiency.

It seems that it might be more appropriate for my application. I
should expect the core to work well for any HF transmission line
transformer I'd like to wind, as long as I don't saturate the thing,
right?

I've seen a lot of tuner baluns made with powdered iron cores which
don't give enough impedance for a decent balun of any kind. And the flux
density will be much greater in a 4:1 voltage balun like you have now
than in a 1:1 current balun. Type 43 ferrite is a good compromise choice
for a 1:1 current balun. If you want to try using the core, put a bunch
of turns on it and measure the impedance with an antenna analyzer if you
have one. You'll need about 500 - 1000 ohms to make an effective balun
for an approximately matched antenna. It might not be possible to get
enough impedance for it to work well on all bands with a multiband antenna.

Roy Lewallen, W7EL

Roy,

Take a look at http://home.earthlink.net/~christras...k4to1Balun.pdf
for a supposed 4:1 current balun on a single hole torroid. I have no
first hand experience with this design.

73,
Larry, W0QE



Roy Lewallen wrote:


I don't know of any way to make a 4:1 current balun with a single
one-hole core. I think it's possible with a two-hole (binocular) core if
done right.

Whoops, my mistake. A single binocular core which obviously has 2
holes. Sorry

Larry, W0QE


Larry Benko wrote:

Roy,

Take a look at http://home.earthlink.net/~christras...k4to1Balun.pdf
for a supposed 4:1 current balun on a single hole torroid. I have no
first hand experience with this design.

73,
Larry, W0QE



Roy Lewallen wrote:


I don't know of any way to make a 4:1 current balun with a single
one-hole core. I think it's possible with a two-hole (binocular) core
if done right.

I also made a mistake in that I failed to qualify my statement. I meant
only transmission line transformers. A conventionally connected
transformer will act as a current balun, and any ratio can be made with
a single core. However, it's difficult to get the extreme wide band
qualities from one that you routinely get from a transmission line
transformer. I don't agree with Trask that his design is a transmission
line transformer despite the two holes, but it does seem to have very
good bandwidth. Also, although he looked at the return loss with various
output terminals grounded, I don't see any measurements showing how well
it actually balances the output currents. But most conventional
transformers do a good job of that, so this one probably does.

Roy Lewallen, W7EL

Larry Benko wrote:
Whoops, my mistake. A single binocular core which obviously has 2
holes. Sorry

Larry, W0QE


Larry Benko wrote:

Roy,

Take a look at http://home.earthlink.net/~christras...k4to1Balun.pdf
for a supposed 4:1 current balun on a single hole torroid. I have no
first hand experience with this design.

73,
Larry, W0QE



Roy Lewallen wrote:


I don't know of any way to make a 4:1 current balun with a single
one-hole core. I think it's possible with a two-hole (binocular) core
if done right.

Thanks for the input guys.

I think that I'm going to break out the '259B and check antenna
impedances on various bands first. I'm probably going to go with the
1:1 choke balun even if the common mode impedance on all bands is not
significantly higher than the feedpoint impedance. I guess it
shouldn't be worse than what I'm matching now.


I think that I don't actually want a 4:1 transformer, as I know there
are a couple of bands where the impedance is fairly low, and if I had a
functioning 4:1 current balun, I'd be transforming it even lower before
the tuner can take a crack at it, and that just seems
counterproductive.

I'm living with some degree of imbalance now (which I've tested in that
the antenna SWR is different if I reverse the legs on the "balun"
output... they aren't equal lengths, they're whatever I can throw out
with a slingshot and a weight after a windstorm)

A 1:1 true current balun on some bands and a
not-quite-effective-current-balun on others is probably better than
what I've got now...

73,
Dan

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