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