the size of radiation box for filter desige (hfss)

hi,

everybody, i have a problem for a filter desige. I designed a filter in HFSS, but I found that radiation box size effet the s parameter a lot. my filter works at 1.8-2.2GHz BW. so When lamda=300mm/1.8, S parameter works good, but when Lamda=300mm/1.5, S parameter changed a lot, and there is too much loss in this case.

so is there anyone can help me? which one is right, and how the radiation box effet the simulation results?

Thanks and best regards!