hfss question

question:

2 antennas in an environment such that the air box associated is causing insufficient memory problems and crashing. the air box is currently defined as a radiation boundary at each of its faces.

however i am not really concerned with radiation patterns merely isolation between the two antenna. i can not define any symetry planes due to the nature of the antenna. is there a way to define the air box outside of radiation boundary such that the S parameters will be calculated correctly?

thanks in advance

ghb