If you want to delve into the physical nature of the "dimensional
resonance" in ferrites, this PHD thesis by Glenn Skutt is probably not a
bad place to start. It's pretty dense with equations, but the reference
list is very extensive. Rudy Severns (aka N6LF) is mentioned in the
acknowledgement section and his name appears in several of the references:
https://vtechworks.lib.vt.edu/items/ea42c327-ddc2-4ebd-94bb-1ed9eab600ac
It seems that the combination of high permeability and high dielectric
constant can make the wavelength in the material very short. Thus, a
core cross section of a few centimeters can be on the order of a
half-wavelength in the megahertz range. As a result of this shortened
wavelength there is an affect on the distribution of magnetic flux in
the ferrite cross-section that is analogous to the distribution of
current in a conductor (aka the skin effect). That is about all I have
been able to absorb.
73, Mike W4EF.................
On 12/6/2024 10:24 AM, Jim Brown wrote:
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
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