a question about HFSS

hi, i use HFSS 9.2 to simulation a horn antenna .When i set up radiation boundary by two methods: 1. radiation boundary contain the whole horn antenna (include wave port). 2. radiation boundary only contain the aperture. i find the result by two methods are different .
which is right?
Thankx

hi ChowChen;

the second method is more right compare to first method.
the best method is that you draw a rectangular box (if the horn is recangular) or cylinderical box (if the horn is conical) with the same cross section as the horn aperture and then you define only the sides wall of box as radiation boundary.

so

1-it is better that your radiation box begin from aperture or alittle before it.
2-if you draw a box with a larger cross section than aperture surface it is better you define the cross section as perfect E.

regards.

i have a new question
if radiation boundary only contain aperture,then the current of wall will not be calculate.it will effect the field. Its effect is big?

i didn't say when you do according to second method your results are absolutly correct. and i didn't say the first method is absolutly wrong.
in fact, all methods have some error but which is more accurate?

when you select a face as a radiation boundary, the software supposes that the radiation from that face must happen whether it is correct or wrong. so it is better that you do a compromise between infinit ground plane approximation and approximation of negligable radiation.
you should draw a box with the the same cross section as aperture surface if you want to use infinit ground plane approximation, and should draw a more large box if you don't want to ignore radiations below ground plane.
i think infinit ground approximation is better.
regds.