On Sat, 22 May 2010 16:46:58 +0100, Steve Hunt wrote:
>Jim,
>Thanks for that. Your explanation encouraged me to go and re-read your
>paper. As always it was a "good read". Did I see some significant
>changes from previous versions:
>1) Earlier versions of the paper stressed that 1:1 current baluns wound
>with bifilar windings rather than coax would suffer significant losses
>and heating because of flux leakage - I believe 30%-40% was mentioned.
>The paper encouraged us to experiment by building a 1:1 balun with a
>bifilar winding and noting the temperature rise when operating into a
>well matched load. It was a view which always drew a lot of "flak" if I
>ever quoted it on a forum.
>All of that material seems to be missing from the latest version of the
>paper. In fact it seems to say something quite different - that bifilar
>wound chokes don't suffer high losses and that "When wound with parallel
>wires, the core sees the sum of flux from currents of opposite polarity;
>the differential components cancel, leaving only the common mode flux
>(due to the imbalance in the system)."
>That seems to be a sea change - or did I misunderstand?
You did not misunderstand at all. My earlier statements about high losses
with differential flux were WRONG, and based on some careless experiments I
did more than ten years ago before I started working with ferrites in any
serious way. Based on questions from others, I recently set up careful tests
with bifilar wound chokes on several types of cores and found losses due to
differential flux to be VERY small. What I learned that I was mistaken about
is that there is virtually no LEAKAGE FLUX -- that is, VIRTUALLY ALL of the
flux from one winding couples to the other, so flux in the core perfectly
cancels for a differential signal.
>2) I believe the section on current baluns with impedance
>transformations other than 1:1 is new. Earlier in the paper you warn
>quite strongly that the flux in the cores of voltage baluns is directly
>dependent on the differential-mode signal; I was surprised not to see
>the same warning relating to current baluns in that new section.
>Again, perhaps I misunderstood .... it has been known ;)
No misunderstanding at all, It is new. I simply did more work and learned
more things about how these things work. That's one of the great benefits of
publishing things -- when you miss something or get it wrong, people tell you
about it and you learn something in the process. :) I've always liked to say
that "he who does nothing does nothing wrong."
If we define a voltage balun as one that couples power from one winding to
another by transformer action through the core (that's my definition), then
ANY voltage balun will produce heating at frequencies where the core is
lossy. #43 and #31 cores are lossy at all frequencies above the AM BC band,
so they will cause heating if used in a voltage balun. #61 cores are pretty
low loss below about 10 MHz, but start getting lossy at 15 MHz and above. You
can see that in the graph of u' and u'' on the Fair-Rite data sheet for each
product -- u' represents series inductance, u'' represents series resistance.
The beauty of impedance transformation by series/parallel wiring of common
mode chokes (the Guanella balun) is that they do NOT put differential flux in
the core like a voltage balun. Each core DOES see the flux, but it shows up
across the high Z common mode parallel resonance of the choke, so there's
very little current (and thus very little heating) if the Z is high enough.
73,
Jim Brown K9YC
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