how to build infinite ground for a slot antenna using HFSS?

Hello,
recently, i have encountered a slot antenna design problem.
As all you know, slot antenna is a complementary structure of a dipole.
When simulate a dipole, mesh is built for only a dipole.
On the other hand, in the case of a slot antenna, mesh is built for whole ground plane.
Since I am assuming that the slot antenna is perfectly complement to a dipole, the ground plane then becomes infinite. So in this case, too many mesh built up, causing out of memory.

Although I checked the option indicating the infinite ground plane button at the PEC boundary,
then the gain of the slot antenna is observed to be very low, approximately -40~-50 dB.
I think such value is not reasonable.

So I ask you to give me other options to set up infinite ground plane for the slot antenna.
If doing so using HFSS is impossible, please give me the options with other tools.

Thank you