Robert Thain g0hgw wrote:
>I've managed to lay my hands on some ferrite cores....
>How can I work out what they are made of and what
>their permeability is ?
1. Download the Fair-Rite catalog from <http://www.fair-rite.com/>.
It contains detailed specifications of many, many types/sizes/shapes
of materials/cores/beads/rods/etc. Your cable beads are most likely
made of type 43 ferrite. The Fair-Rite catalog will show you that,
and many other types. Other manufacturers make similar products with
different labels.
2. Get a vector impedance bridge such as the Autek RX Vector Analyst Model VA1.
3. Measure the complex impedance of a test winding of one, two, or
five turns, depending on frequency.
4. Match what you find against the characteristics given in the
Fair-Rite catalog for a bead of the same dimensions as yours. I'd
look at type 43 first.
5. The Fair-Rite catalog also includes formulas relating the
inductance of a bead directly basic dimensions and the permeability
of the material.
6. Be aware that every ferrite has a "turnover" frequency, which
differs from one type of ferrite to another, above which it looks
more resistive/conductive than magnetic or inductive. To determine
the magnetic permeability you must measure below this frequency.
Above this frequency, all ferrites tend to look the same. E.g.,
types 43 and 77 ferrite look the same in the higher HF range, but
they look very different (77 has much greater permeability, so a
winding has much greater inductance) down in the MF range.
73 de Chuck, W1HIS
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