Coil32 inductance calculator

A recent online posting gave unequivocal recommendation for the Coil32 Inductance Calculator for application to a ferrite toroidal HF current balun.

Always interested in these things, I tried to download it to evaluate it but there was rigmorol to create an account and aware that I have never downloaded a calculator that handled that specific problem at all well, I was reluctant.

They do however have some online calculators that are supposed to use the same algorithms and methods as the downloadable software, so lets review the one for a toroidal ferrite inductor.


Above is the data entry form, and warning bells sound. The “releative magnetic permeability” field is a simple scalar quantity, but the permeability of most ferrites at HF needs to be considered as a complex value (ie having real and imaginary components).

screenshot-07_11_16-16_28_04Above is their documentation of the formulas used, they are based on Al which is based on Initial Permeability, the permeability typically at 10kHz where permeability is approximately a real number.

For example, relative permeability for the common #43 mix at 10kHz is 800 (+j0), whereas at 7.1MHz it is 332-j228.

The suitability for use at RF usually depends much more on complex permeability at radio frequencies than it does on µi at say 10kHz.

cf01Above is a plot from the Fair-rite data book showing the complex permeability characteristic of #43 ferrite material.

Clearly inductance and impedance calculations based on µi are good only up to about 700kHz.