seeding mesh on the radiation boundary in HFSS

Dear Sir:

I am doing a simulation on antennna by HFSS9.2. I found if I change the max length of mesh on the boundary, the radiation pattern will change and it will bring little change to S11. So how long should we define the mesh on the radiation boundary?

Thank you!!!

Regards,

HFSS people say to choose boundary at minimum dustance of lambda by 4 ...the mesh length can be done using lambda by 6.....the more smaller u go...more accurate the result but more is computation time...
so lambda by 6 is better...