radiation box dimensions in hfss

am confused about the role of radiation boundary dimension on the simulation results on a patch antenna. can anyone provide me the way to calculate the dimensions of radiation box for a patch antenna in hfss. its really creating problem for me.

if you mean by the air box that you create for far field region with radiation boundry assigned to it......then its length and width would be same as that of the ground and subtrate....and its height would be above the ground plane about lemda/8 to lemda/12.....its up to you what u want to use for yopur best results.....

i am also designing a microstrip patch to operate over 1.5GHz.....but i just cant get the desired result.....so can u help me with your design....