ads input impedance

In ADS designGuide, there is a file called Stab_vs_freq_pwr. It uses a simplecoupler to simulate the transistor impedance during the runtime, without breaking the circuit. But there is no description or example how to use it. The schematic is as follows.

Does anyone know how to use it? thanks!1

You see b1 and a1 on the ideal coupler...
These are reflected and incident waves to the transistor.
Reflection coefficient(RC)=b1/a1
Input impedance=1+RC/(1-RC)
simple..

Thanks BigBoss!
I agree with your equation but can you tell me how to use your give equations in ADS?

If a1 and b1 are complex quantities, the only thing you should do is to write in an equation in dds page..

Now I can use your equation to obtain the transistor's source impedance, but if I want to know the impedance looking into the power source side, how should I do?
Thanks!

Added after 51 seconds:

I mean, if the input side is unknow, for example, in the interstage matching, I want to know the impedance of both side.