Brian, would you consider the N2PK VNA in the "fancy" category rearding
your calibration comment?
Tks
73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com
On 1/23/2026 6:27 PM, Brian Beezley wrote:
AC0C said:
"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."
Whether the Y21 method is needed depends entirely on the test fixture.
Because it's more convenient to use the VNA's built-in S21 method, I
recommend generating an .s2p file for the S-parameter plotter and
check the difference. If the error is tolerable, use S21 in the VNA.
I forgot to mention that both the S21 and Y21 methods assume the port
impedances are 50 ohms. NanoVNA calibration compensates for any
deviation from 50 ohms for port 1 but not for port 2. Here's an
example of the wide deviation of the port 2 impedance for a NanoVNA-H4:
https://k6sti.neocities.org/vnaz
The impedance error will directly affect the results.
Recent firmware for the NanoVNA-H and NanoVNA-H4 offers partial port
impedance compensation with something called Enhanced Response. It
seems quite good. You can download the latest firmware here:
https://github.com/DiSlord/NanoVNA-D/releases/
If you have a fancier VNA that measures S11, S21, S12, and S22 and does
12-term calibration, any port 2 impedance error will be calibrated away.
Finally, a common-mode choke is a distributed device. Common-mode
errors can arise due to coupling to the VNA cables and the VNA itself.
You can minimize the problem by not handling or touching the VNA while
making a measurement. Use the plastic stylus that came with it. Don't
connect a USB cable during measurement. Using the Y21 method helps
reduce CM as well.
Brian
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