I think You can't simulate the large reflector antenna in EDA's like HFSS or MWS. FDTD and MoM needs too many memory and time to simulate so large structures. I work on large reflector antennas several years and I don't know any good EDA tool for this problem. As I remember, FEKO can simulate such antennas in GTD approximation, but this method inaccurate and has many limitations. Most of reflector antenna designers creates the personal code for such design.
Regards,
Kit