Far field profile of grating

Hello,

I am trying to simulate the far field of a grating. In my simulation I am using periodic boundy conditions and a port exitation. When I am cacluting the far feild profile, it seem that it did not take into account that my stracture is perioidic.

Is there any way of taking into account finat\infanat mumbers of periods in the Far fiel simulation?

Thank you very much!!!