GRASP has established itself long ago as the industry standard for precise modelling of reflector antennas. It builds on decades of research and development in numerical methods for solving EM problems, and the thorough testing by the demanding satellite industry vouches for accuracy and efficiency.
Many R&D projects have contributed to new features in GRASP, a great deal of them led by the European Space Agency (ESA). This provides the versatility needed by the antenna engineer to not only model the primary radiation from a reflector system, but also to account for imperfections such as surface errors, support struts, cut-out regions, scattering in surroundings and other antenna systems, just to mention a few.
GRASP is available for Windows and Linux operating systems. You can try it out in advance by downloading the free Student Edition.