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Momentum with multiple ports

Question asked by Leyo on Feb 17, 2014
Latest reply on Feb 18, 2014 by volker_muehlhaus

May be a very basic question on ADS momentum. ADS version used is 2012.08.
I have matching structure used to match LDMOS drain (Attached as  Two_port). This is a wider and thinner tracks as shown in the attached file. Two ports are defined P1  and P2. When momentum simulation is done, at 1.8 GHz S11 is 17-J20.18. Simulation is done in the schematic view with this copper structure used as layout component. S11 in the schematic view is also 17-J20.18.

In the real circuit, I would like to add some capacitors on the wider copper. So I defined another port P3 (Attached as Three_port). Now the EM simulation gives S11 as 17-J8.18. Why S11 is  different from the previous case? I haven't changed anything on copper??
I simulated this in schematic view with layout component. Without the third port terminated, S11 is 18.3-j20.7 and third port terminated 17.3-J9.07 (Close to Momentum simulation value). Please help me understand this.

In my circuit, I might add two or three caps to match the LDMOS. But, if add more  ports and  don't connected anything on the port in the schematic view, the results are different. How to handle this type of situations?