Some time ago I published a calculator for estimating the impedance of RF inductors on toroidal ferrite cores (Calculate ferrite cored inductor).

The calculator (input form above) use the core dimensions and complex permeability as the basis for calculation.

There are some popular cores that are not simple toroids and so not suitable for direct use with that calculator. For these cores, a practical method is to measure the inductance constant Al (inductance of 1 turn in nH) at low frequency (where µ=µi), and using the µ’,µ” characteristic from the datasheets, to calculate the impedance at the desired frequency. Note that µ’,µ” are usually frequency dependent for ferrite materials.

Above is an example of the use of the new calculator (Calculate ferrite cored inductor (from Al)) on a binocular balun core. (Al was determined by measurement of inductance at 10kHz with an RLC meter to be 90µH with 6t, so Al=90000/6^2=2500.)

Above is a plot of estimated Z using this method.

Note there are lots of inductor calculators that base their estimate on Al, few if any adjust for the actual complex permeability at the frequency of interest, nor of the effect of self capacitance at this one does.

Last update: 26th March, 2015, 10:38 AM

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