I use my Nanonvna with an HP 8721A directional coupler. As noted above attach ch0 to the input of the coupler and ch1 to the reflected port of the coupler. Attach a short to the directional coupler's output port and do a through cal with the short in place. S21 is now the return loss of what is connected to the coupler's output. Because the Nanovna also outputs S21 phase you can calculate the impedance of what is connected to the coupler's output port. The HP 8721A has a directivity spec of 44 dB from 100 kHz or so to 110 MHz. I have verified this with an HP cal kit load. This method might even be more accurate than using Ch0 making an S11 measurement. If you don't have a good commercial coupler, a very good one can be made. All it takes is some wire and a pair of ferrite toroids. Look up the Stockton coupler on the web.
