On 7/12/2021 1:34 PM, David Hachadorian wrote:
It's quite interesting working with the choke calculator, but I notice
that the Rs values differ significantly from K9YC's Choke Cookbook. For
example, with a Fair-Rite 2.4" #31 toroid, on 28.4 MHz, the calculator
predicts that 10 turns of RG-400 has Rs=0.8K, and the cookbook shows 4.4K.
Hi Dave,
Ferrite common mode chokes are parallel resonant circuits, formed by
stray (parasitic) capacitance between turns, inductance and resistance
coupled from the core. That capacitance will depend on the transmission
line used for the winding -- it's dimensions, winding style, and
dielectric properties. These concepts are discussed tutorially in
k9yc.com/RFI-Ham.pdf, with more in the text that is part of the Cookbook.
Measurement of chokes is non-trivial. The test fixture must be carefully
designed and calibrated for the measurement, and the choke must have
"zero length" leads so that lead length does not shift the resonance.
My Cookbook is based on measurement of hundreds of chokes, all of them
wound on four cores at my measured tolerance limits of the 100 or so
2.4-in o.d. and 4-in o.d. cores in my stash. Measured complex Z for each
ham band was collected in a spreadsheet for each "limits" core, and for
each choke, the published value of Rs is the worst case (lowest) value
for each choke.
I haven't studied this calculator, but in general, such calculators will
produce wildly inaccurate results for chokes around resonance unless
capacitance and resistance are taken into account. Even for widely
spaced turns, there is inter-winding capacitance through the core as the
dielectric. All of my designs are for tightly spaced turns.
73, Jim K9YC
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