[TowerTalk] Test Fixture for Common Mode Chokes

[Jeff Blaine]

It looks like the Y21 method is a bit better, especially with increasing 
frequency, but the S21 is a decent proxy as is mentioned on the web page 
article already.  And a real indictment of S11...  Wow.

I bought one of those gadgets as well after seeing the video - because 
it makes the process of getting setup to do the test quick and easy and 
that's a real benefit here.  When I'm looking at a CMC on the VNA, most 
of the time I'm wanting to compare two chokes and as long as the 
measurement setup is consistent I can do that even spread over time.  
The other part is I'm looking for the relative Z vs. frequency in the 
case of a wider-banded application which makes me wonder what sort of 
OSL implementation there is. But I'm guessing that the lengths & strays 
involved over HF ranges is going to be minimal.  Seems like it will be a 
handy addition to the little setup here.

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com

On 1/23/2026 5:05 PM, Brian Beezley wrote:
> AB7E said:
>
> "Any thoughts on this text fixture?"
>
> Dave, use the Y21 method instead of the conventional S21 method to 
> suppress any shunt capacitance in the test fixture. This S-parameter 
> plotter implements both methods. Scroll down to see a comparison of 
> results for the two methods for a common-mode choke:
>
> https://k6sti.neocities.org/sp
>
> K6JCA's explanation and derivation of the Y21 method:
>
> https://k6jca.blogspot.com/2020/07/the-y21-method-of-measuring-common-mode.html 
>
>
> Brian
>
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