[RFI] Ferrites for < 500 khz

[Jim Brown]

I just studied both the pdf you referenced, then looked them up in the 
Fair-Rite catalog.  This is a MnZn ferrite material, and when used for 
suppression, is using ONLY the dimensional resonance -- the clue is that 
the resonant frequency does not move with more turns, but Z is 
multiplied by the square of the turns.

Remember that for suppression, resistance is far superior to inductance, 
because inductance can be cancelled by the capacitive reactance of a 
cable that is shorter than a quarter wavelength. The curves show a 
rather low Q (broad) dimensional resonance in the range of 2 MHz, with 
enough resistance to be useful between about 700 kHz and 4 MHz for the 
largest core -- 2675540002.

The good news is that you can increase the choking Z without changing 
the resonant frequency, so it should be no trick to get 10K ohms at the 
resonant peak with perhaps 8-11 turns, depending on the size of core used.

There's a conceptual discussion of all of this in k9yc.com/RFI-Ham.pdf  
and in other materials on k9yc.com/publish.htm.  The AES paper has a bit 
more of the history and theoretical development, the RFI-Ham more 
practical application, and has the benefit of my ongoing research after 
the AES paper was published in 2005.

73, Jim K9YC

On 9/12/2013 8:40 PM, Michael Germino wrote:
> Read about this a while back.
>   http://www.fair-rite.com/newfair/pdf/LowFreqSuppression.pdf
>   
> I have no experience with these and at the time, couldn't find where you could buy them.
> Are there ferrites for RFI rejection in the 50 khz - 500 khz range? I've been using material #31 and wonder if there is something more useful?  If so, where can I buy it?
>
>