a question about CPW and microstrip simulation @60GHz

Hello, I am a new guy here.
I hope to compare the Q value of microstrip and cpw.
I met a problem when i simulated microstrip (with a shielded ground plane between the alumina layer and the 15Ohm-cm Si substrate) and conduct-back cpw transmission line by ADS Momentum. In the simulation, i have to connect the shielded ground in Microstrip line and the surface ground in CPW to the lower ground plane. Of course, using the ground port in Momentum can define any layer to be a ground reference. But when I tried to use vias which are very close to each other to connect the ground plane, it gave me wrong results of the Q value at very high freqs(>30GHz).
Could you give me some ideas about how to use vias to define a ground layer correctly at very high freqs (by ADS momentum)?

Thank you very much

The ADS does not solve inside the conductor. Try to simulate your structure using HFSS and don't forget to solve inside the conductor.

If possible try to do the same simulation using Sonnet.
I had problems in the past when using HP momentum. I could get more reliable results using sonnet.
If you have access to a license for this software I think it will be worth trying it.
S.

Thank you for your advice~
I will try it by other tools.
By the way, is the quasi-TEM approximation still correct for CPW at 60GHz?