Dear John, LF Group,
Most of these "EMC" cores are made of something similar to Fair-Rite #43
material, with mu of around 850. They will work quite well at LF and MF,
although a given size of core will require about 1.5 times as many turns as
the "traditional" 3C85/90/95 etc materials to get the same inductance.
Unfortunately, the distributors have only heard of ferrite cores being used
to put on cables to supress noise, so they don't know much about materials,
and usually classify according to size, and, if you are lucky, impedance in
the HF/VHF range, which is not very relevant to LF.
You can make a reasonably accurate determination of the permeability by
measuring the inductance of a core wound with several turns to get AL = L /
n^2, The formula relating AL, mu, cross-sectional area, and magnetic path
length is
AL = 4pi * 100 * mu *A/l
with cross sectional area of the magnetic path A in square metres, and
magnetic path length l in metres (in the case of toroidal shapes, this is
roughly the average of the inner and outer circumferences of the core), and
mu is the relative permeability.
Once you know what permeability you have, you can be fairly sure behaviour
will be near to that of a material with similar mu in one of the
manufacturer's catalogues, e.g. Ferroxcube or Fair-Rite
Cheers, Jim Moritz
73 de M0BMU
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