S-param question...

Below is the sample circuit to analyze s-paramater using Tanner Tools.
Tanner Tools don't have the capabilty to analyze S-parameter!

Therefore, we need to create a circuit that is capable to extract the S-parameter component, S11,S12,S21 and S22!

My question is, why do we need 'K' in the circuit?
Is there any other easier techniques that can extract the S-parameter components?

Pls help me...