Reconciling my #52 choke design tool with G3TXQ’s measurements

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.

Screenshot - 09_02_16 , 18_29_42

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).

Clip 122

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:

References / links