Comment on KN5L on balun CMRR – two wire line example

The article Comment on KN5L on balun CMRR dealt with model and measurement of John's coaxial choke in fixture, dealt with first because it is a simpler model. This article builds on that and models the balun wound with a pair of wires.

Above is the subject balun in fixture.

John's schematic shows the balun as coupled coils, but that does not capture the transmission line transformation that occurs in the actual device. Again the test fixture is used without explanation.

To implement a transmission line model of the balun, we need to capture both its transmission line behavior to differential currents, and its choke behavior to common mode current. To do that, two pairs of coupled coils are added at each end of the transmission line to divert the common mode current via the choke elements.

So, now we can reasonably accurately model the transmission line effect and the choke effects.

Above is the revised Simsmith model that implements the schematic above, and compares the model with measured for that balun configuration and test fixture. Again the value of n was tweaked to calibrate the model, and Cse adjusted for good high end tracking.

Above is my reinterpretation of his measurement data.

Looking at the marker values on my chart, they indicate Zx=5640+j4321Ω @ 14MHz, much higher than the expected common mode impedance of the choke. They are of course nonsense, the t2s function is not a valid transformation for John's test fixture.


The test fixture used seems inspired by the theme ‘more complicated is naturally better'. KN5L's model is flawed.


  • Agilent. Feb 2009. Impedance Measurement 5989-9887EN.
  • Agilent. Jul 2001. Advanced impedance measurement capability of the RF I-V method compared to the network analysis method 5988-0728EN.
  • Anaren. May 2005. Measurement Techniques for Baluns.
  • Skelton, R. Nov 2010. Measuring HF balun performance in QEX Nov 2010.