s parameter

S11 is input reflection coefficient.
which you are measuring at port 1(Term 1)
It indicates the impedance at transistor's gate to source terminal.
S22 is output reflection coefficient measured at port 2 (Term 2)
It indicates output impedance of transistor.

In this figure
The The gain of the amplifier is 3 dB
S11 is 0dB so no reflection
S22 is 0dB with 180 degree phase shift
means your signal at amplifier output will be 180 phase shifted.

I hope this clears the doubt :)

Thanks for replying.

But I simulated it in ADS, the S11=0.33 and s21=0.96, why?
the results may be different when interchange the resistor and AMP.

http://www.zen118213.zen.co.uk/RFMic...Parameters.pdf
Read this for your reference.
You need to understand S parameters first.
They will change when device will change.
when you add or remove resistor it will obviously change device s parameters.

Just remove R1, it is already inside PORT1.
Mazz