With a "normal" nanoVNA (50 ohms), if, during the SOL calibration, instead of using 50 ohms during the LOAD you place a load of 75 ohms, it works. The central point of the Smith will correspond to 75 ohms. On the other hand, it will be necessary to make a rule of 3 for the values calculated by the NanoVNA. It is only the measured ROS that is correct and the Smith only displays this ROS.
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F1AMM
François