CST eigen solver for open-boundary case

Hi, everyone. I have some questions about the MWS-CST eigen solver for open-boudary case.
From the example of microstrip line eigen simulation sample in CST 2005, the top wall is assigned to PEC boundary instead of open boundary. I am wondering whether it is correct enough for higher-order modes?
Since in this case, CST do not use PML boundary such as in HFSS, I think it assume that if the air distance higher enough, the radiation field will decay and can be ignored. But does this setting include some pesudo-higher order modes that do not exist in real world?

Any body know this answer?
I found that in HFSS port only simulation, I will face the same problem for pesudo-eigen mode. It use 2D PEC boundary for solving eigen function.
I am a little confuse that whether I should shrink the port size or enlarge it to get a more accurate result?

For CST eigen mode solver, you cannot find the modes of open structures. Also, if you place it very far from the microstrip line, you will start to have some higher order WAVEGUIDE modes, that is the shield acting as a waveguide cavity. So, I think you should have some feeling where would the resonance frequency be and adjust the shield size for it.