[TowerTalk] New tool - toroidal ferrite core inductor calculator

[Jim Brown]

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