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Message-ID: <172D99CA495841DC87784DBC9160975A@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <200901291143090013.08822D48@smtp.wanadoo.fr>
In-Reply-To: <200901291143090013.08822D48@smtp.wanadoo.fr>
Date: Thu, 29 Jan 2009 22:03:31 -0000
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Subject: LF: Re: Ferrite toroid cores using HEM series materials
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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