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Common mode choke design tool

The technique given at A method for estimating the impedance of a ferrite cored toroidal inductor at RF can be used for the design of common mode chokes that use toroidal ferrite cores.

This article offers several spreadsheets for design of chokes using #43, #52, F14, and L15 ferrite mixes, all of which are potential candidates for a general purpose HF balun for use with an ATU. The spreadsheets contain examples of conventional multi-turn chokes, and the #43 sheet contains a model of a W2DU choke using 25 cores.

The spreadsheets allow specification of an equivalent shunt capacitance (Cs), which has a marked effect on the performance for most practical configurations. Typical values of Cs range from below 1pF to more than 3pF for untidy construction, metal enclosure, coax and crammed cores. Input cells are yellow, and green cells are calculated results and should not be edited.

The spreadsheets use VBA macro functions which you must permit, and complex number functions from the Analysis ToolPak add-in that comes with Excel.

If you build a prototype and measure its impedance with a VNA or other device, the spreadsheet model can be calibrated by adjusting Cs to obtain the same self resonance frequency (ie where X crosses the zero axis) as the prototype.

The calculated Zcm can be plugged into an NEC model of an antenna system (as a load element), and common mode current and power dissipation will be calculated by the model.


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