Bifilar Wound Balun Transformer

I am learning a bit about antenna design and one of the references I
found talks about coupling the antenna to the feedline with a bifilar
wound balun transformer. I dug a bit and although I found any number
of references talking about bifilar wound baluns, none explained clearly
why it is important to be bifilar wound.

Any pointers?

Rick

On 11/3/2012 5:15 PM, rickman wrote:
> I am learning a bit about antenna design and one of the references I
> found talks about coupling the antenna to the feedline with a bifilar
> wound balun transformer. I dug a bit and although I found any number of
> references talking about bifilar wound baluns, none explained clearly
> why it is important to be bifilar wound.
>
> Any pointers?
>
> Rick

Ah, a picture is worth a thousand words. I finally found a page that
shows a bifilar balun in the application circuit I would be using it
with and it makes perfect sense now. Well, mostly. The circuit is
single ended to differential coupling.

I get why the thing is wired up the way it is, I suppose it is important
to use a bifilar winding to keep the two windings as identical as possible.

Actually, I've looked at too many pages. I had two pages mixed up. I
see the one that showed a toroidal core matching transformer is not the
same page as the one that said to bifilar wind the balun. Seems the
first one is a transformer like I'm used to seeing, but the bifilar
wound balun is used in a different way that can't match impedance over
the range I believe the toroid is doing.

It's pretty amazing how many web pages there are that cover in such
detail so many highly specialized topics! And most of these are hobby
pages!!!

Rick

On Nov 4, 8:15*am, rickman <gnu...@gmail.com> wrote:
> I am learning a bit about antenna design and one of the references I
> found talks about coupling the antenna to the feedline with a bifilar
> wound balun *transformer. *I dug a bit and although I found any number
> of references talking about bifilar wound baluns, none explained clearly
> why it is important to be bifilar wound.
>
> Any pointers?

A balun is actually a transmission line transformer.

http://en.wikipedia.org/wiki/Balun

The twisted pair that constitutes the bifilar winding is a
transmission line, with a particular characteristic impedance which
depends on the diameter of the wire involved and the thickness and
natire of its insulation.

IIRR a twisted pair twisted out of enamel-insulated transformer wire
has characteristic impedance in the ball-park of 120R.

Google throws up a few tutorial papers

http://home.earthlink.net/~christras...LTTutorial.pdf

http://www.highfrequencyelectronics....TraskPart2.pdf

Transmission line transformers keep on working to much higher
frequencies than conventional transformers - the inter-winding
capacitance becomes part of the transmission line rather than a simple
parasitic load - and in fact only start falling over when the
wavelength of the frequency being transmitted approaches the length of
the winding.

And - for John Larkin's benefit - this is electronics.

--
Bill Sloman, Sydney

"Bill Sloman"

> Any pointers?

A balun is actually a transmission line transformer.

http://en.wikipedia.org/wiki/Balun

The twisted pair that constitutes the bifilar winding is a
transmission line, with a particular characteristic impedance which
depends on the diameter of the wire involved and the thickness and
natire of its insulation.

IIRR a twisted pair twisted out of enamel-insulated transformer wire
has characteristic impedance in the ball-park of 120R.


** For clarity, it needs to be said that twisting of a pair of parallel
wires in incidental to their operation as a transmission line. Twisting
merely serves to eliminate radiation and pick up of external EM fields.

A "bifilar wound " transformer may well have no twisting of the wires at
all, but simply has them laid side by side in smooth layers.


.... Phil

"Bill Sloman" <(E-Mail Removed)> wrote in message
news:f3d0bcb5-3ca3-4a83-9445-(E-Mail Removed)...
> A balun is actually a transmission line transformer.

Not a necessary construction method; a balun is just a transformer with
tapping such that it inverts one side.

> The twisted pair that constitutes the bifilar winding is a
> transmission line, with a particular characteristic impedance which
> depends on the diameter of the wire involved and the thickness and
> natire of its insulation.
>
> IIRR a twisted pair twisted out of enamel-insulated transformer wire
> has characteristic impedance in the ball-park of 120R.

I got closer to 30 ohms last I measured a pair. Enamel is a whole lot
thinner than extruded jacketing. It's going to be even lower in a
piled-up winding due to the crowding.

The low frequency way to think of it: your leakage inductance is almost
exactly the inductance of the windings as a transmission line.

If you take a piece of twisted pair 1m long, it'll have maybe 0.5uH
inductance (measured at one end of the pair, shorting the far end, at a
frequency well below the electrical length of the line). If you wind it
up onto a form with an air core (making a bifilar solenoid, say), the
self-inductance of each winding might be a few uH, while the inductance
between wires remains the same (it's lower, if anything). Note that you
can measure this leakage two ways: terminus shorted (as a transmission
line) or secondary shorted (transformer leakage). The difference is, you
test P1-S1 and short P2-S2, or test P1-P2 and short S1-S2.

Now if you insert a permeable core, inductance goes way up (into the mH,
perhaps), and coupling coefficient likewise goes up (some fraction less
than 1.0). But leakage remains fairly constant.

Leakage depends almost entirely on winding construction. Self-inductance
depends on the windings and core. Coupling coefficient is the factor
relating the two.

(Yes, you can make a transformer that specifically depends on core
geometry, not just winding construction. An example would be two coils at
right angles, with a core snaked through each. Without the core, they
have zero mutual inductance (infinite leakage). With the core, it's
nonzero. I'm more interested in applications where you actually give a
damn about performance in the first place. )

The important thing about transmission line transformers is to forget
about using them as transformers. Use them as transmission lines! If you
put a few loops of coax on a core and drive the shield (calling the shield
the primary, P1-P2), you can't expect any useful kind of behavior from
that, because the shield carries all sorts of crazy currents, depending on
how it's looped through, and which turns it's adjacent to, etc. If
instead you drive the transmission line from one end (P1-S1), you'll get
the same signal out (P2-S2), delayed, except the core allows you
common-mode voltage. You could flip the terminal end around (S2-P2), and
get an inverted signal!
http://www.picosecond.com/product/pr...asp?prod_id=47
That's more or less what they do here. The shield necessarily does still
carry a signal (the act of flipping the terminals forces the output
voltage onto the shield anyway), but this occurs "after" the signal
propagated through, and what you do with the shield is now an open
variable -- you could loop it through a whole bunch of ferrite beads,
damping out any oscillations.

It follows that you can create any ratio by connecting transmission lines
in parallel, looping them through a core (it doesn't even matter that the
same core is used, it's just a common mode choke now!), and connecting any
desired series-parallel combination on input and output sides to set the
desired impedance and ratio.

The dirty secret of transmission line transformers is, they aren't at all
interested in reducing leakage inductance, or capacitance, or anything
like that. It's just a big common-mode choke that lets you pipe signals
from wherever to wherever else.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

On Nov 4, 10:50*am, "Tim Williams" <tmoran...@charter.net> wrote:
> "Bill Sloman" <bill.slo...@ieee.org> wrote in message
>
> news:f3d0bcb5-3ca3-4a83-9445-(E-Mail Removed)...
>
> > A balun is actually a transmission line transformer.
>
> Not a necessary construction method; a balun is just a transformer with
> tapping such that it inverts one side.

In the sense that the original source of the name was as a contraction
of "balanced to unbalanced transformer".

The wikipea article makes it fairly clear that one should understand
it as a transmission line transformer. As Phil Alison correctly points
out, you don't actually have to twist the wires together to make them
into a transmission line, though twisting them is a mechanism which
does keep the pair close together.

> > The twisted pair that constitutes the bifilar winding is a
> > transmission line, with a particular characteristic impedance which
> > depends on the diameter of the wire involved and the thickness and
> > natire of its insulation.
>
> > IIRR a twisted pair twisted out of enamel-insulated transformer wire
> > has characteristic impedance in the ball-park of 120R.
>
> I got closer to 30 ohms last I measured a pair.

How thick was the wire? The thickness of the enamel is more or less
independent of the copper gauge, and the impedances is going to be
appreciablyb higher for 40# gauge wire than for 10# gauge.

>*Enamel is a whole lot thinner than extruded jacketing. *It's going tobe even lower in a
> piled-up winding due to the crowding.

Most of the field is confined between the two wires of the pair. I
wouldn't think that adjacent wires would make much difference.

> The low frequency way to think of it: your leakage inductance is almost
> exactly the inductance of the windings as a transmission line.
>
> If you take a piece of twisted pair 1m long, it'll have maybe 0.5uH
> inductance (measured at one end of the pair, shorting the far end, at a
> frequency well below the electrical length of the line). *If you wind it
> up onto a form with an air core (making a bifilar solenoid, say), the
> self-inductance of each winding might be a few uH, while the inductance
> between wires remains the same (it's lower, if anything). *Note that you
> can measure this leakage two ways: terminus shorted (as a transmission
> line) or secondary shorted (transformer leakage). *The difference is, you
> test P1-S1 and short P2-S2, or test P1-P2 and short S1-S2.
>
> Now if you insert a permeable core, inductance goes way up (into the mH,
> perhaps), and coupling coefficient likewise goes up (some fraction less
> than 1.0). *But leakage remains fairly constant.
>
> Leakage depends almost entirely on winding construction. *Self-inductance
> depends on the windings and core. *Coupling coefficient is the factor
> relating the two.
>
> (Yes, you can make a transformer that specifically depends on core
> geometry, not just winding construction. *An example would be two coilsat
> right angles, with a core snaked through each. *Without the core, they
> have zero mutual inductance (infinite leakage). *With the core, it's
> nonzero. *I'm more interested in applications where you actually give a
> damn about performance in the first place. )
>
> The important thing about transmission line transformers is to forget
> about using them as transformers.

<snip>

This is wrong. I've certainly used them as 1:1 isolating transformers
and they worked fine.

> It follows that you can create any ratio by connecting transmission lines
> in parallel, looping them through a core (it doesn't even matter that the
> same core is used, it's just a common mode choke now!), and connecting any
> desired series-parallel combination on input and output sides to set the
> desired impedance and ratio.

There is some interesting literature on creating integer ratio
transmission line transformers, and if you are clever enough I'm
fairly sure that you can create non-integer ratios - I think there's a
famous paper on the subject. There are also a lot of ways of getting
it wrong.

> The dirty secret of transmission line transformers is, they aren't at all
> interested in reducing leakage inductance, or capacitance, or anything
> like that. *It's just a big common-mode choke that lets you pipe signals
> from wherever to wherever else.

That's certainly one way of using them.

--
Bill Sloman, Sydney

"John Larkin" <(E-Mail Removed)> wrote in
message news:(E-Mail Removed)...
> We do exactly that in a bunch of products, namely use the shield as a
> primary winding and the inner as the fully isolated secondary of a
> transformer. We do 1:1 and 1:2 (voltage step up) at levels from 5
> volts to over 100.
>
> https://dl.dropbox.com/u/53724080/Circuits/Xfmrs.JPG
>
> This makes a transformer with very low leakage inductance, so we get
> sub-ns rise times into a 50 ohm load.

Except that, as I said, the leakage is not particularly low. One gets
better performance in that regard from, say, copper foil pairs (which,
ultimately, is still doing the same thing, but with a low impedance
symmetrical stripline, not 50 ohm coax). Which is often done in power
circuitry. But "very low leakage" is not what you're going for, so it's
best not to claim that's what you're doing.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

On 11/3/2012 9:28 PM, Jeff Liebermann wrote:
> On Sat, 03 Nov 2012 17:15:27 -0400, rickman<(E-Mail Removed)> wrote:
>
>> Any pointers?
>> Rick
>
> If you're connecting to an antenna, this might be helpful:
>
> A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing
> <http://audiosystemsgroup.com/RFI-Ham.pdf>
> Quoting:
> The primary function of most baluns, at least in our ham
> stations, is to minimize the interaction of our antennas
> with the transmission lines that connect them to our radios.
> There's more to baluns than just impedance matching.
>

Wow, that's a lot of reading. Thanks.

Rick

On 11/3/2012 9:42 PM, John Larkin wrote:
> On Sat, 3 Nov 2012 20:40:37 -0500, "Tim Williams"
> <(E-Mail Removed)> wrote:
>
>> "John Larkin"<(E-Mail Removed) > wrote in
>> message news:(E-Mail Removed)...
>>> We do exactly that in a bunch of products, namely use the shield as a
>>> primary winding and the inner as the fully isolated secondary of a
>>> transformer. We do 1:1 and 1:2 (voltage step up) at levels from 5
>>> volts to over 100.
>>>
>>> https://dl.dropbox.com/u/53724080/Circuits/Xfmrs.JPG
>>>
>>> This makes a transformer with very low leakage inductance, so we get
>>> sub-ns rise times into a 50 ohm load.
>>
>> Except that, as I said, the leakage is not particularly low. One gets
>> better performance in that regard from, say, copper foil pairs (which,
>> ultimately, is still doing the same thing, but with a low impedance
>> symmetrical stripline, not 50 ohm coax). Which is often done in power
>> circuitry. But "very low leakage" is not what you're going for, so it's
>> best not to claim that's what you're doing.
>>
>> Tim
>
> But it works.

See, this is the sort of stuff that, if I were a potential customer,
would turn me off to doing business with you. Geeze, if I am talking to
someone about what is going on in a system and they say to me, "but it
works", I would think they didn't understand it at all.

Do you not see how your posts make you look?

Rick

On Sat, 03 Nov 2012 18:50:54 -0500, Tim Williams wrote:

> The important thing about transmission line transformers is to forget
> about using them as transformers. Use them as transmission lines!

Anybody know how to accurately model a transmission line transformer in
Spice, taking into account core properties?

--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
(Richard Feynman)