10 GHz siw slot antenna

Hi I want to design siw slot antenna in 10GHz but I can't put slots in broad wall of the waveguide because the surface current is different with air field waveguide as show in the picture
the with of the structure is a=18.6mm b= 1.57mm Er= 2.33 and amplitude distribution is schebyschef and offset of the slot is less than usual because they must be from 1 to 4 mm
can you help me How can I do that?
plz send me some method if you know Engi_boy@yahoo.com