I guess this is why R&S introduced the T-check many years ago as a simple approach to establish quality of measurement
In my own simple words, L calibration establishes the center of the smith chart, O and S define the boundaries so using a fourth known load such as 25 ohm (being substantially different from the 3 calibration loads and easy to create using two calibration loads and a Tee) shows the accuracy in one direction This for sure does not eliminate the amplification of errors when moving away from Z0 but at least gives a understandable indication of quality.
For the nanoVNA this has proven very valuable as the calibration was perfect till 900MHz but the T-check (or a measurement of 25ohm (almost) pure resistance) showed considerable opportunity for improvement.
As the T-check can be done using any impedance it allows any user to establish a sense of measurement accuracy at the used impedance.
It is my strong wish to translate thave this lengthy thread translated into something practical a average nanoVNA user is able to understand. For that reason I was asking about the choice of the 4th load to ensure the % error in the cross ratio has a real meaning. "substantially independent" is not usable for the average user. Is 25 ohm substantially different? A pure capacitor but at what reactance?. It would be of benefit to the community (that has to endure the many mails on this subject) to have simple guidelines that can be applied easily with relevant meaning
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NanoVNA Wiki:
/g/nanovna-users/wiki/homeNanoVNA Files:
/g/nanovna-users/filesErik, PD0EK
