How to set Absorbing Boundary condition in HFSS?

Hi all,
The first time to simulate antenna using HFSS.And i do not know how to define the size of air box(Absorbing Boundary condition) so that i can get a correct result.I simulated 7.55GHz patch antenna,i set the different the length,the width,and the height,got differnt 3-D polar.Anybody kindly tell me which rule we should obey?
Thank you

Hi:

The air box should be 2-3 lambda (with lambda calculated from the central frequency) above the patch. The bottom part really does not matter if you have a ground-plane. Then select each single face of the air box and assign radiation boundary.

Hope this helps.

use PML boundaries with lamda/4 distance. that will reduce ur problem size too

Thank both of you,what you told is helpful to me.