also I'm curious to know is there any way to change between 2D to 3D or vice versa by some setting ?
your question is not clear. Momentum uses Method of Moments and gives a full, accurate solution of the 3D fields for planar problems: metal and vias with stacked dielectrics. The only simplification in that math is that substrates are assumed to have infinite size.
If you need to model arbitrary 3D geometries, or finite dielectrics, you can use FEM. There is a switch in emSetup to change between FEM and Momentum (RF mode, microwave mode). FEM requires a separate license feature.
volker_muehlhaus do you know is there any way we can find characteristic impedance of transmission line(for which I have created layout in ADS Momentum) ?, instead of trial and error method (simulating multiple times by some random terminations for which we get least loss). If yes, kindly share.
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