hfss ground pec

Hi,
I am currently working on a similar project.I've used a small rectangle lumped port to feed the monopole against the infinite ground plane.The infinite groung plane is a perfect E-boundary with the the infinite ground plane option checked.I have used a regular polyhedron for the monopole. The length of the monopole is a little less than lamda/4 to achieve resonation.So far the radiation pattern is not satisfactory (not omnidirectional).That's all for now.