I went through the dozen or so .s2p files I have for common-mode chokes.
The error due to shunt port capacitance was most often restricted to the
resistive and reactive parts of the measured impedance, not the
impedance magnitude. This means that the pi network formed by unwanted
shunt port capacitance in conjunction with the choke is acting like an
all-pass network. I have no idea why.
Due to shunt port capacitance, the conventional S21 series measurement
method overestimates effective choke resistance. After using the the Y21
method to cancel the capacitance, choke resistance at HF typically drops
less than 10%. I think this puts the error in the negligible category
for most hams. I have a few measurements that go to 50 or 60 MHz. Error
at VHF is quite a bit higher. It may not be negligible for a 6m choke.
If anyone with a VNWA wants a script for the Y21 method, I can provide
one developed by DH0KAI. It lets you plot Y21 results right in the VNWA
software, including shunt capacitance at each port. This lets you rework
and improve your choke fixture without the need for a capacitance meter.
Using all four S-parameters provides best Y21 accuracy. You also need
them to obtain separate capacitance values for both ports. If your VNA
only provides S11 and S21, you can interchange the port cables and make
a second measurement. Combine the two [S11 S21 0 0] files to [S11 S21
S12 S22] with https://tinyurl.com/45xk3zum.
I'm always looking for .s2p common-mode choke files. I seem to learn
something from each one I examine. Please forward any you may have.
Thanks to NA6O who sent me several.
I've added Y21 shunt capacitance plots to my S-parameter plotter. You
can see some curves here:
https://k6sti.neocities.org/sp
Brian
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|