868MHz IFA antenna design in CST

Hello,
I design 868MHz IFA antenna. It should be placed in one corner of 160x160mm PCB. Small part of GND under the antenna structure is destinated for antenna GND. On the rest part of PCB will be elements and also GND. So can I set whole surface of PCB as a metal (copper)? Are the simulations done OK? What did I wrong?
I attach my design and results.
868MHz.rar

Can somebody check my design?

Without having CST: Ground under the IFA sounds wrong ... there should be no ground below the IFA.

But this is not printed IFA. That is the structure:

Ok, this should work. But from my experience, increased distance between antenna radiator and ground gives an antenna impedance that is easier to match. Have you tried a cutout in the ground below the antenna?