Simulation of the reflection coefficient in different materials (CST Studio Suite)

Hi,
I am trying to simulate the reflection coefficient of an Bow Tie Antenna with CST Studio Suite. I want to simulate the reflection coefficient for different materials like water or a phantom tissue, which simulates biological tissue. Can anyone tell me, how I can change the properties of the surroundings of the antenna in CST Studio Suite? I already designed the antenna and I can run the simulation, but i do not know how to change the properties of the surrounding material.
Thanks

If problem is just to change the background material the answer is very simple
CST>Simulation>Background (It is on the left most corner)
And from here you can assign the material of your choice from CST library.
By default it is PEC