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.

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

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

wrote:
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...

To the extent that the balun is working, it will work with complex
impedances as well as with a purely resistive ones.

The Trask paper has several designs for 2 core 4:1 current baluns that
should work. They need two cores. If you want really wide band
performance, you can stack a low frequency and higher frequency
ferrite toroid for each of those cores. With this approach you can
achieve a 1000 to 1 frequency range. The long tube bead ferrites make
good baluns in a small space.

Insert "ratio" and repeat after me -

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

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/

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

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

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