Alan,
Thanks for the additional info. I understand what you're doing there, and can easily put the circuit in SimSmith and play with it.
However, the original poster (Andy) indicated an interest in measuring impedance of an end-fed half-wave (EFHW) antenna, which we know will be 2000-4000 ohms at resonance (and harmonics thereof), being fed at the voltage antinode.
Andy,
I'd probably try a little broadband 9:1 unun to step it down to within range of the nano, as that should be fairly accurate, easy to implement, and easy to understand. But now I'm wondering if nano (with 9:1 unun attached) could be could be calibrated with open, short, and 450-ohm load, placing the unun behind the calibration plane and removing its irregularities; and also, how the various parameters might be converted...
Tiny 9:1 ununs, with sma and screw terminals, are available for a few $ from several rtl-sdr vendors such as nooelec and rtl-sdr.com.
73, --kv5r
