Solution frequency in hfss ?

Hi guys!
Before I start I like to appreciate your help in advance :)
I'm trying to design a PIFA antenna for 915Mhz ,,, obviously before i finish my design, during the design procedure My resonant frequency is different each time.
SO WHAT SHOULD I SET THE SOLUTION FREQUENCY ? because result would change when i change it ... I have a frequency sweep from 700Mhz to 1500Mhz and solution type is terminal.

Thanks