Paul, David, Alan, Jim, LF, (and anyone I may have left out)

Many thanks everyone for your replies and very useful information.  What kicked 
my request off was the need to make an input transformer for the Softrock for 
136.

I have a few of these cores (and also assorted cores inherited from one of our 
club members in Suffolk) and will do some measurements.  

73
John F5VLF

*********** REPLY SEPARATOR  ***********

On 29/01/2009 at 22:03 James Moritz wrote:

>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