I am designing a conical horn antenna at 60GHz to be used as a feed antenna for a parabolic reflector. I have calculated all the dimensions w.r.t. 60 GHz (especially the rectangular waveguide part) but my S11 looks messed up. In other words, why don't I have a notch at 60GHz and have good return loss around all the frequency range?? I have attached the S11 graph, far-field gain graph and parameters of the antenna.
The S11 in general is pretty small; what values are you expecting? By "messed up", do you mean you are expecting larger S11 values away from 60 GHz? What do you expect the bandwidth to be, given the cutoff frequencies of the various modes?
I would check to see if the antenna is actually radiating: plot the realized gain and some of the fields inside the antenna.