how big should the air be drawn in HSFF antenna simulation

I am always confused that the size of the air "box" in HSFF antenna simulation.

I have tried different size of air "box" and they resulted in different simulation results.

Anyone could suggest some guidelines in this issue?

Thanks a lot

Take a look at this tutorial:

http://www.emtalk.com/tut_1.htm

They also compared the HFSS results with measurements:

http://www.emtalk.com/forums/viewtopic.php?t=7

The distance should be larger than λ/4 theoretically, but you can assign it as large as possible.

HFSS recommends minimum distance of λ/4 between your antenna outer structure and the airbox. This was from one of the tutorials. If you make it too large, your computation time will increase a lot.

thanks all!

I tried airbox height of λ/4 at first but height=λ improved my model's characteristic closer to the fabricated antenna.