[Amps] Amps Digest, Vol 264, Issue 7

[Jim Brown]

On 12/6/2024 9:52 AM, John Lyles wrote:
> Powered Iron makes low loss toroids but are not useful as EMI 
> suppressors. The inductance and the net impedance is low also. As we all 
> know, ferrite works very well if you get the right mix.

Ferrites work for RFI suppression BECAUSE of their parallel self 
resonance, and the chemistry of each mix determines both where that 
resonance occurs and it's usefulness in suppression. NiZn chemistries 
(for example, Fair-Rite #43, #52, #61) provide a single resonance, 
usually fairly high-Q; MnZn chemistries (Fair-Rite #31, #75, #77, #78) 
provide two, one based on windings, the second based on cross-sectional 
area of the flux path. Fair-Rite's #31 is unique -- it's dimensional 
resonance in convenient sizes lands in a sweet spot for HF IF, and ONLY 
IF, turns are wound through it.

There's a lot more of what I've learned about this over 20 years of 
study in k9yc.com/RFI-Ham.pdf  and in the 2018 Choke Cookbook at 
k9yc.com/publish.htm  I began this study in 2004 to address RFI to large 
sound systems and published it as an AES Paper in 2005. I first 
published RFI-Ham.pdf, which addresses its applications to ham radio, in 
2007.

73, Jim K9YC