SOS!!!　 S21 VS AC Gain?

LNA simulation in ADS, The AC simulation uses the standing wave voltage only. But the S-Parameter simulation uses output power divided by incident power(V and I). How to understand it?


AC gain= 20*log(mag(vout)/mag(vin))
S21=b2/a1
simulation results:
S21=2* AC gain (dB)? confused!

The source impedence is different 50ohm vs 0ohm.

why?
what is the difference between AC gain and AC gain?

sorry, i mean:

what is the diference between AC gain and S21?