On 10/13/2020 8:12 AM, John Oppenheimer wrote:
Hi Jim,
For high power Balun Applications, most critical parameter is Ferrite
core loss which causes core heating. Core loss, parallel resistance
(Rp), can be computed from complex permeability data from Fair-Rite.
Hi John,
While I respect your engineering chops, you clearly don't understand how
ferrite common mode chokes work. They DEPEND on the loss component
coupled from the ferrite core, which has a broad peak around resonance
of the winding. Fair-Rite's #31 material is MnZn, which exhibits two
resonances. One is formed by the inductance of the winding and the
capacitance between turns; the second is a dimensional resonance that
results from standing waves within the core. #43 material is NiZn; these
materials do NOT exhibit dimensional resonance, because loss is low at
the frequencies where it would occur.
At HF, the two resonances in multi-turn chokes on #31 cores combine to
provide a much broader resonance, comparable to what happened in
stagger-tuned IFs. This has two implications. First, the broader
resonance allows the choke to cover more bands.
Second, and more important, ferrite cores have wide manufacturing
tolerances that cause the resonances to shift in frequency. The broader
impedance curve formed by chokes on #31 material allows a designer to
specify chokes that will work within a specified frequency range with
that tolerance variation; it is NOT possible with the much narrower
impedance curve of chokes wound on #43 or #52 material.
My "choke cookbook" is the result of first characterizing nearly 200 #31
cores of a given size, selecting cores at the tolerance limits for each
size, winding and measuring more than a thousand chokes on these
"limits" cores. The cookbook is based on chokes working with all of
those "limits" cores.
After seeing recommendations from a ham in the UK for #52 material, I
bought 40 of these cores over a period of about four weeks, splitting
the order between four franchised vendors (that is, ten from each
vendor). I characterized those cores, selected cores at limits, and
wound chokes using that ham's recommendations. I could not reproduce his
results -- the tolerance variation moved their resonance away from their
intended operating range.
I learned about dimensional resonance from a classic engineering book on
ferrite applications by E. C. Snelling that a colleague found for me in
the engineering library of the U of Chicago, where he was on faculty.
Snelling's work is considered "the bible" by engineers working in mfg
and application of ferrites. It's reference 7 in the AES paper, and is
also referenced in the tutorial.
These concepts are discussed in detail in an AES paper from 2005 and in
a tutorial I wrote for hams in 2007, the latter updated several times
over the years.
http://k9yc.com/AESPaperFerritesASGWeb.pdf
http://k9yc.com/RFI-Ham.pdf
The "cookbook" is here. http://k9yc.com/2018Cookbook.pdf
73, Jim K9YC
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