Guilin Sun
Ansys Employee

A1: If you think it is mandatory to include the whole substrate in the simulation, you will need significant simulaiton time to count on the multi-reflections. Since the substrate is relatively thick, I think it is reasonable to siulate a small portion of it. The lower air/substrate interface merely introduces small refleciton loss.

I have illustrated how to do this in the screenshot and description.

A3: please refer the screnshot to expand the substrate in periodic directions.