MFJ Tuner "Current Balun" conversion.
 

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

MFJ Tuner "Current Balun" conversion.
 

MFJ Tuner "Current Balun" conversion.
 

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.

MFJ Tuner "Current Balun" conversion.
 

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.

MFJ Tuner "Current Balun" conversion.
 

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.

MFJ Tuner "Current Balun" conversion.
 

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

MFJ Tuner "Current Balun" conversion.
 

An important correction: I DO NOT get a lower-than-50ohm *impedance* on
any band.

I may be taxing the limits of my '259B but this is what I measure as
far as ballpark magnitude of impedance.

80M 225 ohms
60m 600 ohms
40m 630 ohms
30m 330 ohms
20m 420 ohms
17m 206 ohms
15m 216 ohms
12m 216 ohms
10m 180 ohms

Nowhere is this resistive.

So, maybe the 4:1 current balun is the more appropriate one. How does
the impedance transformation work with reactive loads?

Here are the impedances if anyone needs:
80m 41-j220
60m 500-j320
40m 500+j370 when paralleled with a 1k resistor (300+j1000ish?)
30m 39-j325
20m 300+j255
17m 50-j200
15m 190+j104
12m 70-j205
10m 170+j60

Will a 4:1 current balun transform a random impedance Z1 to Z2 where
|Z2|=|Z1|/4? I know Roy pointed out that this won't work for a voltage
balun...

73,
Dan

MFJ Tuner "Current Balun" conversion.
 

MFJ Tuner "Current Balun" conversion.
 

Roy Lewallen wrote:
wrote:
. . .
Will a 4:1 current balun transform a random impedance Z1 to Z2 where
|Z2|=|Z1|/4? I know Roy pointed out that this won't work for a voltage
balun...

No, it won't either. The amount of error with any balun type depends on
the balun construction. You'll just have to build one and see what it
does.

It would be interesting to know what your antenna Z looks like on each
band at the input to the existing balun. That is, what do you get when
you connect your antenna to the balun output and the analyzer to the
balun input?

Since the balun is connected to a tuner, inaccurate impedance
transformation won't matter much. The only concern would be if it's a
sign of some other problem in the balun.

For a monster 4:1 current balun of the type that has two independent
'cores', see page 30 of:
http://www.yccc.org/Articles/W1HIS/C...S2006Apr06.pdf
(or for short: http://tinyurl.com/qnzs3 )

The whole article is about common-mode chokes, and is well worth
reading. It shows what can be done to tame a "noisy" QTH, if you're
prepared to go to extreme lengths... and this guy certainly was!


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

MFJ Tuner "Current Balun" conversion.
 

Ian White GM3SEK wrote:
Roy Lewallen wrote:
wrote:
. . .
Will a 4:1 current balun transform a random impedance Z1 to Z2 where
|Z2|=|Z1|/4? I know Roy pointed out that this won't work for a voltage
balun...

No, it won't either. The amount of error with any balun type depends
on the balun construction. You'll just have to build one and see what
it does.

It would be interesting to know what your antenna Z looks like on each
band at the input to the existing balun. That is, what do you get when
you connect your antenna to the balun output and the analyzer to the
balun input?

Since the balun is connected to a tuner, inaccurate impedance
transformation won't matter much. The only concern would be if it's a
sign of some other problem in the balun.


Well, yes and no. Good tuner designers go to a lot of trouble to
maximize the Q of the inductors to minimize loss. It's doubtful that the
Q of the balun inductance is nearly as good, so loss is liable to be
higher if the balun is contributing a significant amount of reactance.
Otherwise, I agree, it doesn't make much difference. The balun might
move the impedance to a point where the tuner can't match it, but it's
just as likely that it'll move an otherwise unmatchable impedance to
within the tuner's range.

Roy Lewallen, W7EL

MFJ Tuner "Current Balun" conversion.
 

A choke balun has NO impedance or turns ratio. It is silly to refer to
one.

To make one just wind 15 or more turns of twin speaker wire on a 2"
diameter, one-hole, ferrite core.

A one-hole core is a ring with a hole in the middle.

Ferrite permeability need not be high. 200 or 300 is good enough and
will provide enough inductance to cover the 160m band. Low
permeability materials also have lower loss at the higher frequencies.
Not that a choke balun is a lossy component. Efficiency is extremely
high.

Because the length of wire is only about 1/8th of a wavelength at 30
MHz it will be ok at that frequency too.

All the talk about saturation is so much hot air. You couldn't
saturate it even if you tried. The currents in the two wires run in
opposite directions and cancel each other out.

It has a a lower loss and higher power-handling ability than a core
wound with the usual very small diameter coax.
----
Reg.

MFJ Tuner "Current Balun" conversion.
 

Reg Edwards wrote:
A choke balun has NO impedance or turns ratio. It is silly to refer to
one.

Consider me silly as well as an "old wife". A choke balun has common
mode impedance, and that impedance is its single most important quality.
If the common mode impedance isn't adequate, it won't perform its
function. It can be measured by short circuiting the input conductors
together and output conductors together to temporarily make one
conductor, and measuring the impedance between the ends.

. . .

All the talk about saturation is so much hot air. You couldn't
saturate it even if you tried. The currents in the two wires run in
opposite directions and cancel each other out.

I agree that saturation isn't a problem, but disagree about the reason.
Core flux density is a function of the common mode current, which is in
the same direction in the wires and doesn't cancel out. The objective of
the balun is to minimize this current, but in a high power system even
with an effective balun, the I^2 * R loss, where I is the common mode
current, can still get large enough to make the core hot. However, if
you use a high-permeability, low frequency ferrite, the flux density
will still be way below saturation even when the core is hot enough to
break.

. . .

Roy Lewallen, W7EL

MFJ Tuner "Current Balun" conversion.
 

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

MFJ Tuner "Current Balun" conversion.
 

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

MFJ Tuner "Current Balun" conversion.
 

MFJ Tuner "Current Balun" conversion.
 

Insert "ratio" and repeat after me -

"A choke balun has no impedance (ratio) or turns ratio."
----
Reg.

MFJ Tuner "Current Balun" conversion.
 

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/

MFJ Tuner "Current Balun" conversion.
 

Ian... extreme lengths is right! I like the ferrite cores strung onto
the 240V lines.

I'm sure this approach would help if I wasn't on a graduate student
budget and living in an apartment (though maybe I could sneak around
and install snap-on ferrites on everything?)

After reading that article and thinking back a bit, I think a real
current balun would be worthwhile.

A previous incarnation of my "invisible" antenna used *one* length of
magnet wire to a SO-239 tuner center pin and used the balcony rail
connected to the ground on the tuner.

Switching to the "balun" and two legs reduced my electric(al/onic)
noise quite a bit. I think at least trying to enforce balance is
worthwhile. I guess I need to be prepared to spend some money if I
want to be able to choke off common mode currents on all bands .
I certainly am trying to tame a noisy QTH, but I've made some progress.
As is often the case, it was mostly *my* stuff causing the noise.
Still, I've got a few persistent sources. I know *some* of it is
radiated and I'm sunk there. My 6m antenna is a moxon rectangle with a
string of 60 or so #43 beads as a balun mounted on a fiberglass mast.
Everything it's picking up is radiated :-)

Roy, I'll do the impedance measurement on the balun input, just as a
matter of curiosity, and post the results.

First pass, I think, will be the 1:1 current balun, especially if the
MFJ balun's core is of a worthwhile material. I'll let the tuner do
its job.

I'm curious about what would happen with the 4:1 current balun, butI'll
have to order some cores.

-Dan

MFJ Tuner "Current Balun" conversion.
 

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

MFJ Tuner "Current Balun" conversion.
 

The imaginary part of these numbers seem a bit odd (oh no, only three
of them are odd--the rest are even). Is your antenna resonant on any
band?
I'm probably not well-versed in all the impedances an antenna/feedline
can take at the tuner, but would like to know what antenna and feedline
this is.
73,
Glenn AC7ZN

MFJ Tuner "Current Balun" conversion.
 

My antenna is not resonant on any amateur band. It gets almost there
around 4.1MHz.
(SWR 1.8:1 or something) It's an unequal leg length variously bent
doublet made with whatever length of 30 gauge magnet wire I managed to
fling out into the trees on that particular occasion.

Approximately: one leg 60 feet long about 24 feet high at the feed,
about 35 feet high at the other end. The other leg about 50 feet long,
bent groundward at a point 35 feet up and 30 feet out with an almost
vertical descending portion.

The "feedline" is a slightly tapered open wire section of the same
magnet wire with more or less 1.25 inch spacing. It's only about 4
feet long.

The antenna tuner is hidden in a big blue wooden box on my balcony
directly under the antenna feed, so I don't need much of a run of
feedline. I set plants on it.

The length numbers are *very* approximate. I've paced out the
tree-balcony distance and also checked with google earth, but I figured
a tape measure might be a bit suspicious.

I actually have very little trouble tuning the antenna. I do have a
lot of noise and removing all I can of the conducted component would
serve me well, I think.

73,
Dan
N3OX

wrote:
The imaginary part of these numbers seem a bit odd (oh no, only three
of them are odd--the rest are even). Is your antenna resonant on any
band?
I'm probably not well-versed in all the impedances an antenna/feedline
can take at the tuner, but would like to know what antenna and feedline
this is.
73,
Glenn AC7ZN