Differential flux leakage in a Guanella 1:1 balun

This article has been copied as reference for a new article from my VK1OD.net web site which is no longer online. The article may contain links to articles on that site and which are no longer available.

I have been asked by a correspondent to comment in the context of my model of a Guanella 1:1 balun wound on a ferrite toroid (Duffy 2008a) on the impact of differential flux leakage as discussed in the ARRL 2011 Handbook on the predicted losses in a Guanella 1:1 balun using a ferrite toroidal core


The ARRL 2011 Handbook (Silver 2011  20.23) states [i]f the line is made up of parallel wires (a bifilar winding), a significant fraction of the flux associated with differential current will leak outside the line to the ferrite core. Leakage flux can exceed 30% of the total flux for even the most tightly-spaced bifilar winding.

This might suggest that differential current will contribute significantly to balun core losses and consequently transmission loss. The claim is made without explanation or substantiation, or without making conclusions about any resultant loss. This is the makings of fear, uncertainty and doubt (FUD), and hardly the enlightenment that readers might expect.


(Brown 2008) makes a similar assertion: [t]he ferrite core of a transformer balun (nearly all voltage baluns) sees all of the transmitted power, but the ferrite material that surrounds a feedline sees only the leakage flux (a few percent of the transmitter power with a coax feedline, and 30-40% in a balanced feedline)! and briefly describes an experiment to demonstrate the effect.

Setting aside the issues in this statement not directly relevant to the flux leakage issue, the experiment as far as it is described is questionable, and there are not results reported.

However, Brown recants the flux leakage position in a posting to the Towertalk online forum (Brown May 2010) :

>/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
/>/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."

Hats off to Brown for identifying and ‘fessing up to a problem in his earlier work.

(Brown Jun 2010) does not contain the errant material, it is silently missing.

However, the ARRL Handbook 2011 (Silver 2011) seems to have drawn heavily on Brown as a source, possibly a single source as they are wont to do, and the 30% flux leakage assertion has been published as fact without explanation or substantiation.


In developing the mathematical model of a Guanella 1:1 balun on a toroidal ferrite core (Duffy 2008a), the model results were validated as best as could be done with a ham grade VNA (Ten-tec VNA). It is certainly not a laboratory instrument, but sufficiently good to identify significant departure in measured transmission loss from the transmission line model part which is essentially based on the RF Two Wire Transmission Line Loss Calculator (Duffy 2008b) for the two wire line construction. Note that the particular VNA, and perhaps most will not directly display transmission loss, and it is a mistake to interpret S21 which is displayed, as transmission loss.

This is not to say that leakage is zero, or that there is zero resultant core loss, just that it has not been apparent in prototype measurements.

Link / References

  • Brown, J. 2008. A ham's guide to RFI, ferrites, baluns, and audio
    interfacing R2.
  • ———. May 2010. [TowerTalk] Baluns/tutorial/notes. http://lists.contesting.com/pipermail/towertalk/2010-May/117677.html (accessed 20/10/2010)
  • ———. June 2010. A ham's guide to RFI, ferrites, baluns, and audio interfacing R5a.
  • Duffy, O. 2008a. A model of a practical Guanella 1:1 balun. https://owenduffy.net/balun/Guanella/G.1-1.htm accessed (20/10/11).
  • ———. 2008b. RF Two Wire Transmission Line Loss Calculator.  https://owenduffy.net/calc/tl/twllc.htm (accessed (20/10/11).
  • Silver, H Ward  ed. 2011. The ARRL handbook for radio communications. 2011 ed. Newington: ARRL.


Version Date Description
1.01 20/10/2011 Initial

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Owen Duffy 1995, 2014. All rights reserved. Disclaimer.