improving S11 for horn antenna using HFSS

hi all,
i have designed horn antenna fed by rectangular as well as square waveguide separately. the S11 obtained is very bad at my operating freq,720 GHz. im getting only some -6 or -7 dB. i increased the airbox,still no change in S11,also tried renormalising the impedance to the waveguide impedance. eventhen no change.can anyone tell me how to improve it.also my 3d polar plot is very bad(it has so many ripples). please suggest me some idea to improve both.

regards,
Mubassira

Something sounds fishy -- a pyramidal horn fed with the fundamental mode of a rectangular waveguide should have a nice impedance match and well-behaved antenna patterns. You can find an overview of horn antenna design in the antenna engineering handbook (Johnson). See the following link for typical E-plane and H-plane pattern cuts and VSWR for commercially available horns.

http://www.ets-lindgren.com/manuals/3160.pdf

Without seeing a screenshot, we can only speculate that there may be a problem with the source or boundaries of the model.

Other problem may also be the port definition. 3D solvers are rather tricky to the way ports are defined.

cheers