Thanks jallem,
you are right, i am simulating a monopole over a perpendicular ground plane. If my monopole is a cyclinder with 1mm diameter, 4 cm long, and the perpendicular plane is 10cm X 10cm, is it larg enough?
What i worry is, i just assign boundary of the perpendicular plane as a finite conductivity (copper), and then draw a coaxial which outer conductor which is connected to the plane and the inner conductor is connected to the monopole, and then excite it by a lump port, without telling the HFSS that my perpendicular ground plane is a finite ground plane, it seems the 3d pattern always assume my perpendicular plance as radiator.
I don't want to assign the plane as infinite ground plane. Just assume i have any shape of metal plane, how can i assign the boundary and excitation so that HFSS know it is a ground plane?
Thanks for helping.