Creating a infinite ground plane in cst

HI I am designing a monopole antenna using cst. I need to use a infinite ground plane for better performance of the antenna. For infinite ground plane I have put Et=0 for boundaries where the ground plane is. But it is not giving me the result. I have heard a process of decoupling plane but I dont know the process.
Can anyone please suggest me how can I design infinite ground plane in cst?

i guess ground plane dimensions over 6h+L, 6h+W acts as infinite groundplane