Graphene patch Antenna

Hi.

I want to simulate patch antenna in band 600 GHz.
The Patch is Graphene not metal.

I have a problem with pattern in farfield, which shows ground plane is radiated not graphene patch.

would you please direct me?

Note:
patch is graphene.
substrate is Sio2 with epsilon 3.9
ground plane is PEC.
excitation is waveguide port. (mode 1)

Thanks