# 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.

## The challenge

A ferrite cored toroidal inductor has important characteristics that make
design a challenge:

- ferrite permeability is a complex value that is frequency dependent; and
- the ‘inductor’ is more completely a resonator.

(1) is dealt with by using the correct complex permeability in calculations.

(2) has little effect at less than say one tenth of the lowest self resonance
frequency, and up to about half that first self resonant frequency can be
modelled reasonably well by a small equivalent shunt capacitance.

This article offers several spreadsheets for design of chokes using
various 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.

As mention earlier, these devices are really resonators and exhibit self
resonances. Up to perhaps twice the first self resonant frequency these effects
can be modelled reasonably well by a small equivalent shunt capacitance.

## A process

So the first step is to carefully measure the first self resonant frequency
of a prototype, carefully meaning to ensure that the test fixture is not
disturbing the thing being measured. Then calibrate the model by adjusting the
value of Cs in the spreadsheet to obtain the same first self resonant frequency
as measured.

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.

## Download

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

The files for various materials can be downloaded at:
Zcm* files .