I have read some posts on this topic, and it appears some NanoVNA users are able to vary the level output to port 0.  Page 34 of the manual implies such.  I have connected my port zero to my scope and varied the output slider using NanoQT sweep parameters and the output level holds steady at about 150 mv no matter where the output slider is placed.  I also tried with no success to vary the output level trying all 4 TX options (stimulus. CW, TX power, 0,1,2,3 directly controlling NanoVNA without PC).  I am attaching my configuation data.  Why am I unable to vary the output ?  Is my VNA (SAA2-N with N connectors made by Zeenko, purchased from AURSINC) a bad clone ?

Hello, my experience is that:
1) the level control is not supported via USB so NanoVNA-QT cannot control output level
2) the level control on Nano GUI (0,1,2,3 levels) only work for ADF4350 output, so it applies to frequencies > 140 MHz
3) it seems there is a bug in CW mode: when you change the ADF level from Nano GUI, the actual output level does not change until you also change the frequency (even a few Hz).
Hope this help. Best regards, Marco.

The only PC based winblows program that I have found that will allow you to change output level is NanoVNA-App by OneOfEleven, great program!!!