HFSS eigenmode solver, minumum frequency

Hi everybody

I have to find dispersion diagram for a periodic dielectric medium over a ground plane. I'm using HFSS v11 and eigenmode solver but I don't know how to set up a good value for minimum frequency. I know that my structure is resonant at about 18 GHz, but everytime I change minimum frequency I got different results from HFSS.

How should I choose minimum frequency in order to have a result I can trust? Thank you