I need the same thing: drawing two half spaces facing each other...the only infinite thing I found was the ground plane, but separating an half space from the other by it means imposing a screen that prevent backscattering of waves.
Another thing Inoted was that the simulator divide the space into two regions, the one said 'space of the problem' in which it computes the fields...etc and the other said 'space of the mesh' in which it generate a mesh without computing anything.The outer is called background...