Assign the material for background in RF module

Hi all,

I am simulating an aperture in RF 3D module. I created a silicon ellipse lens and an aperture, and I wanted to see the far field beam passed through the aperture and the silicon lens. And I want to change the background material to Silicon too. I found the global material is the material valid throughout the model. But is this material also valid for the background (outside of the model)?

Hope someone can tell me how to do it. Thanks very much in advance!