[TowerTalk] Test Fixture for Common Mode Chokes

[Brian Beezley]

This plot shows how the conventional S21 method can overestimate the 
resistive part of common-mode choke impedance:

https://i.postimg.cc/fy30Qk3f/R.png

The Y21 method can plot the shunt capacitance at each VNA port. This 
stray capacitance reduces accuracy when using the S21 method. This plot 
compares port 1 shunt capacitance for similar chokes made and measured 
by different hams:

https://i.postimg.cc/pdtQQQms/C1.png

Using a Y21 capacitance plot, you can adjust your fixture for minimum 
stray capacitance without using an external instrument to measure it. I 
think this is a nifty trick! It will improve accuracy for S21 
measurements you make with no external software to implement the Y21 method.

Brian