ansoft hfss horn antenna
If you are using HFSS8.5 you must draw the horn antenna, assign a perfectH boundary to the apperture of the antenna, draw a box which will include the aperture and a small part of the horn antenna, copy the horn antenna and paste it, then substract the copy out of the box, assign perfectE boundary to the intersection (except for the apperture) of the box and the horn antenna and assign radiation boundary to the outer surfases of the box. Don't forget to put a wave port where necassary. I haven't tried it in other versions of HFSS but I hope it will work. Good luck!