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
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