problem in designing airborne antenna using HFSS Software

im facing problem in designing the airborne antenna using HFSS Software...the main problem which am facing is related to the ports..as the airborne antenna is made up of slotted waveguides, and combining those waveguides into the form of an array the slotted airborne antenna is designed..right now am working on an array(means a single waveguide with 10 uniform slots)..once i make the waveguide and give port to the front face of it, i get a different radiation pattern and once i make the waveguide and give port to the bottom face, i get different rad pattern...the correct rad pattern is observed when the waveguide is excited by giving port to the bottom face but thoeratically its wrong...theoratically the correct rad pattern should be observed once the waveguide is excited by its front face but the pattern which i get by doing this is incorrect...plz if any1 can suggest anything.