Thank you all for the nice info. The puzzle is much closer to completion.
In the mean time I have received the bridge, made first measurements. So far I can see that |S21| using the bridge agree very well with |S11| when connecting the DUT to CH0, which was expected. At least I can calculate the magnitude of DUT's impedance, knowing RL.
I've asked Rune, the author of the wonderful piece of software NanoVNA-saver to add external bridge support, as suggested by Erik (thank you both).
73 Ady YO2NAA
