Background in HFSS is always PEC. If you need other background you should surround you structure with your material and assing radiation boundary condition on its outer surface. But I'm not shure how radiation BC is matched with dielectrics with epsilon <> 1, i.e. not vacuum, is there reflection when dielectric touch boundary? I guess that there is no reflection, I hope... ;-)
Background in HFSS is always PEC. If you need other background you should surround you structure with your material and assing radiation boundary condition on its outer surface. i.e. not vacuum, is there reflection when dielectric touch boundary? I guess that there is no reflection