CST Infinite Substrate Thickness

Hi,
I like to setup an infinite substrate thickness in CST but I am not sure how to do it. I know how to setup infinite ground plane.
The CST help is not indicating anything on this. Please help.
I have attached my file for your view.
BRgds
Manimaran

Why you want an infinite substrate? You're using open boundary condition.

Thank you for your reply.
Well I need to have a substrate that is infinite to fit my derived equation, the simulation is crucial. The file I attached is open add space which I used initially, how can I set the boundary to be Infinite on -Z direction.
I have tried to put periodic boundary but its extend both side +Z and -Z to infinite thickness.
Any idea would very much welcome.