Changing simulation medium

Hello,

can you somehow define simulation material without defining specific 3D model? I wanted to simulate antenna beneath the earth to make a scenario with  "infintie" burial dimensions. I kept using sommerfeld integrals from Construct->Structuers->Planes/arrays->Plane/ground.

Basically is there anyway to fill "white space" (which im assuming is free space with epsilon 1) with specific medium without defining "Big cuboid" with specific material.