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