You can calibrate your measurement reference plane at a number of places. You can do your calibration at the SMA terminals of the NanoVNA. You can do your calibration at the ends of coax cables terminated in SO-239 coax connectors as I have. You can do your calibration at the input or output terminals of your balun and I have done that also. Where you do a particular calibration is called the reference plane. If you want to read the impedance at your balun output, then perform your SOL calibration with your Short, Open, and Load connected to the output terminals of your balun. I have done that for my ladder line fed HF antenna system from 3 MHz to 30 MHz. Here's some further information:
