Software for running PIFA simulation

Hello,
I have designed a PIFA GSM antenna on a FR4 PCB, but i want to run some simulation. Can anyone suggest me some software tool?
Thanks in advance

HFSS

For a PIFA on FR4 the easiest way would be to go to AWR site and download a 30 days trial version of MWO (or you can buy it, but is a lot of money).
The EM simulator is user friendly, predictable, and very accurate.