when simulating a open structure just like microstrip??

the open structure is set in a airbox.
is it necessary to make radiative boundary on the face of airbox?
who can tell me? thank you !

It's depends on the real operating mode of microstrip line. If this line placed in box with metal shield on the top and this shield closest than one wavelength (another criterium also possible) You should consider it in simulation. If this shield far from microstrip or covered the absorber You should use the radiative or PML boundary type.

Regards,
Kit