HELP with waveguide feed to planar antennas in (Agilent)HFSS

Hi,
Can anybody help me out regarding how to define a waveguide feed to a planar microstrip antenna? What are the dimensions of the waveguide and what are the walls of waveguide and other boundry conditions defined as? Any project neone could send will be gr8. (hharnal(at)gmail.com)
Thanks,
Hitesh