David,
with NanoVNA you can not find out, if an antenna (think you called it 'aerial') is goo for your
purpose to receive signals, or not. Depends on your receiver, if it has an input impedance of
50 ohms, you could check, if your antenna has a good SWR or not. But SWR is only a matter
to match input (output) plus cable to the 'other side' = antenna. A good SWR says nothing
about the efficiency of an antenna. A 50 ohm resistor at the end of your coax cable will show
you an SWR of 1:1. This will be a perfect reading of S11, but that's not a good antenna.
To bring it to the point: nanoVNA is not a tool to let you know, if you are having a good and
efficient antenna for your existing receiver and the frequency you want to listen at all..
