Slot antenna Ground Plane problem in HFSS

Hi, everyone

I am using HFSS to design a microstrip fed slot antenna on a finite ground plane at 5GHz. When my ground plane is 50mm x 50mm, I get reasonable gain pattern. However, increasing the ground plane to something bigger distorts the radiation pattern especially at boresight. Further increase in ground plane does not solve my problem. i stopped increasing when i got 'out of memory' error msg from HFSS. Can someone help me out please?

Thank you.

Can you upload your design ?

Your ground plane dimensions are comparable to wave length, thus the simulation result is expected.