A correspondent wrote with concern of the apparent difference between graphs produced by my #52 choke design tool with a graph published by G3TXQ of his measurement of 11t on a pair of stacked FT240-52 cores.

I published a note earlier about my concerns with a similar graph by G3TXQ compared to the Fairrite datasheet, and he reviewed the data, found the error and published a corrected graph.

The corrected graph above might at first glance appear different to my model’s graphs, and the first obvious difference is that G3TXQ uses a log Y scale (which is less common). The effect of the log scale is to compress the variation and give the illusion perhaps that in comparison with other plots, this balun has a broader response.

To compare the two, I have roughly digitised G3TXQ’s graph above and plotted the data over that from my own model (with linear Y scale).

Above are my own model predictions (calibrated with Cs=1pF) and overlaid G3TXQ’s reported measurements of R and |Z|. I have not attempted to calculate X as it is very sensitive to error in digitising R and |Z| when R is almost equal to |Z|.

The reconciliation is quite good, better than one might expect given the simplicity of my model, and manufacturing tolerances and measurement challenges.

The values calculated by my choke design tool above are consistent with those calculated by the following calculators:

- Calculate ferrite cored inductor – rectangular cross section
- Calculate ferrite cored inductor – circular cross section
- Calculate ferrite cored inductor (from Al)

## References / links

- Duffy, O. 2015. A method for estimating the impedance of a ferrite cored toroidal inductor at RF. http://owenduffy.net/files/EstimateZFerriteToroidInductor.pdf.
- ———. 2006. A method for estimating the impedance of a ferrite cored toroidal inductor at RF. VK1OD.net (offline).
- ———. Feb 2008. A model of a practical Guanella 1:1 balun. http://owenduffy.net/files/GuanellaBalun01.pdf.
- Fairrite 2013. How to choose ferrite components for EMI suppression. Fairrite product catalog 17th ed p146.
- Guanella, G. Sep 1944. New methods of impedance matching in radio frequency circuits. The Brown Boveri review.
- Knight, D. 2008. The Self-Capacitance of toroidal inductors. http://www.g3ynh.info/zdocs/magnetics/appendix/Toroid_selfC.html (accessed 28/09/2105).
- Knight, D. Jul 2013. The self-resonance and self-capacitance of solenoid coils. http://www.g3ynh.info/zdocs/magnetics/appendix/self_res/self-res.pdf (accessed 28/09/2105).