Thank you very much for the information - it was just what I was looking for.
I will directly connect the NanoVNA to my input circuit to see how it looks now I know that I will not damage the IC by applying too much power - I am mainly wanting to do an S11 measurement so I can try to get a good match on the RF input.
Currently my circuit shows a 15dB gain when I apply a -30dB signal to the RF input.
My current test circuit (set up on a breadboard until I get a rough design in place) is in balanced mode and I was wondering about the 1.5k and 3k input impedance as I had read about this a few times. I think I will need to re-work my toroids as currently they do a 50 to 1.5k ohm transformation but I may also try redesigning this section and use the method described on page 10 of AN1994 (I will need to change a few details in the calculations as the PDF is for an SA605 which appears to have a higher input impedance at around 5k ohm):
Thanks again for the suggestions - they are really helpful.
