[TowerTalk] Baluns/tutorial/notes.

Steve Hunt steve at karinya.net
Sat May 22 10:51:14 PDT 2010


Thanks for the clarification. As you say: "He who never made a mistake 
never made anything".

I remain puzzled by what you just said about the flux in the cores of 
N:1 Current Baluns. Any analysis I've done shows that the flux in one or 
more of the cores *must* be determined directly by the Differential Mode 
signal. Only in a 1:1 current balun is the flux solely dependent on the 
Common-Mode signal.

A simplistic way of looking at it is to take a 4:1 current balun 
comprising two 1:1 baluns connected in parallel at the input and in 
series at the output. For a differential voltage applied across the 
input of V, the voltage across the load will be 2V. The extra "V" has to 
be developed somewhere, and it is by "voltage transformer" action in one 
or more of the cores.

I sketched the voltages here for a floating load:

Excuse the rough drawing. That example is the most benign situation, 
where the differential-mode signal is split equally between the cores. 
There's V/2 across each core, and that directly drives the core flux.

If you try another load-balance configuration - for example ground the 
"top" of the load - you'll find a much more extreme situation. In that 
example you'll get V across one core and 2V across the other - that's a 
worse condition than a 4:1 voltage balun! Ground the "bottom" of the 
load and you will get V across the top core and 0 across the bottom.

No matter how the load is balanced or unbalanced wrt ground, there is 
always that "extra V" being developed by voltage transformer action on 
one or more of the cores .......

.... I believe ;)

Steve G3TXQ

Jim Brown wrote:
> On Sat, 22 May 2010 16:46:58 +0100, Steve Hunt wrote:
> 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. 
>> 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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