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Bifilar Wound Balun Transformer

Started by rickman November 3, 2012
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=A0am, 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 =A0transformer. =A0I dug a bit and although I found any numbe=
r
> 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/~christrask/TraskTLTTutorial.pdf http://www.highfrequencyelectronics.com/Archives/Jan06/HFE0106_TraskPart2.p= df 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" <bill.sloman@ieee.org> wrote in message 
news:f3d0bcb5-3ca3-4a83-9445-e4c84510488b@v9g2000pbi.googlegroups.com...
> 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/product.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 Sat, 3 Nov 2012 18:50:54 -0500, "Tim Williams"
<tmoranwms@charter.net> wrote:

>"Bill Sloman" <bill.sloman@ieee.org> wrote in message >news:f3d0bcb5-3ca3-4a83-9445-e4c84510488b@v9g2000pbi.googlegroups.com... >> 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.
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. https://dl.dropbox.com/u/53724080/Circuits/T760_pulses.jpg 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/product.asp?prod_id=47
Here's a hardline inverter: https://dl.dropbox.com/u/53724080/Circuits/Coax_Inverter/MVC-229X.JPG https://dl.dropbox.com/u/53724080/Circuits/Coax_Inverter/MVC-235X.JPG https://dl.dropbox.com/u/53724080/Circuits/Coax_Inverter/MVC-232X.JPG https://dl.dropbox.com/u/53724080/Circuits/Coax_Inverter/MVC-234X.JPG Low frequency response sucks because it is, after all, a dead short at DC. It gets better if you run the coax through a few ferrite cores. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On Nov 4, 10:50=A0am, "Tim Williams" <tmoran...@charter.net> wrote:
> "Bill Sloman" <bill.slo...@ieee.org> wrote in message > > news:f3d0bcb5-3ca3-4a83-9445-e4c84510488b@v9g2000pbi.googlegroups.com... > > > 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.
>=A0Enamel is a whole lot thinner than extruded jacketing. =A0It's going to=
be 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). =A0If you wind i=
t
> 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). =A0Note that yo=
u
> can measure this leakage two ways: terminus shorted (as a transmission > line) or secondary shorted (transformer leakage). =A0The difference is, y=
ou
> 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). =A0But leakage remains fairly constant. > > Leakage depends almost entirely on winding construction. =A0Self-inductan=
ce
> depends on the windings and core. =A0Coupling coefficient is the factor > relating the two. > > (Yes, you can make a transformer that specifically depends on core > geometry, not just winding construction. =A0An example would be two coils=
at
> right angles, with a core snaked through each. =A0Without the core, they > have zero mutual inductance (infinite leakage). =A0With the core, it's > nonzero. =A0I'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 an=
y
> 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. =A0It's just a big common-mode choke that lets you pipe signal=
s
> from wherever to wherever else.
That's certainly one way of using them. -- Bill Sloman, Sydney
On Sat, 03 Nov 2012 17:15:27 -0400, rickman <gnuarm@gmail.com> 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. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in 
message news:ffcb985qk94e0cf265odu97o95d90sp1bh@4ax.com...
> 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 Sat, 3 Nov 2012 20:40:37 -0500, "Tim Williams"
<tmoranwms@charter.net> wrote:

>"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in >message news:ffcb985qk94e0cf265odu97o95d90sp1bh@4ax.com... >> 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. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators