Dear WimRFP
Thanks for your reply, it helps me think. The radiation below the ground plane is from -50dB to -80dB, it is quite small. The ground plane has no thickness, I just draw a rectangle as the ground plane.
I tried the similar simulation with single patch. When I draw a circle at the bottom of the ground plane and set it as a lumped port, there is radiation pattern below the ground plane. However, when I used the wave port, there is no radiation pattern below the ground plane. So is it because the lumped port? Could you explain this to me? I am not very clear about this. Thank you in advance.
Best regards!