[TowerTalk] Test Fixture for Common Mode Chokes

[Jim Brown]

I haven't studied David's video, but I consider G3TXQ's test fixture to 
be as good as it gets. W6GJB built one for me that mimics it.

Common mode chokes MUST be measured as a parallel resonant circuit, with 
"zero-length leads," as one of my EE profs put it,  and the fixture must 
be calibrated at the tips of the alligator clips that connect to the 
choke, with those leads physically placed as they would be to connect to 
the DUT. Because Z at resonance is so high, both stray (parasitic) 
capacitance and inductance of the test fixture move the resonance. I 
calibrate the fixture using a very short and thick "short,' and an SMT 
1% 50 ohm resistor with short leads soldered on.

My only fault with Steve's work, and it is a critical one, is that his 
choke designs failed to take manufacturing tolerances into account. The 
problem with that is that the NiZn materials have such high Qs compared 
to Fair-Rite's #31 material, which is a very special MnZn material, 
whose characteristics DO allow repeatable designs IF those tolerances 
are taken into account. My designs were done after measuring nearly 100 
#31 cores, selecting four at the limits of their complex 
characteristics. When I did the same with 40 #52 cores, purchased over a 
month or two from four franchised industrial vendors in lots of 10, I 
could not repeat his designs.

73, Jim K9YC

On 1/23/2026 1:47 PM, Tony Brock-Fisher via TowerTalk wrote:
> I think there are issues with that, and I previously always did it 
> according to G3TXQ (SK) method:
> 
> http://www.karinya.net/g3txq/chokes/s21.pdf