modelling large signal analysis anf some linearity question

hi guys, i wish to model my circuit in large signal analysis so as to study for IIp3.
SO Is using volterra series analysis a good method to use?

In addition, i do some simulation for my linearity. I found when i simulating i got a better linearity in exchange a poorer S11, for which eventually leads to poorer S21.

Is that normal for my above observation?

Please tell me, experts?

With regards
novice