If I was looking for help on an antenna design my starting point would be to discuss the RF and physical constraints. The application and so on is also important. Knowing the frequency and bandwidth are essential. Size (3D), weight, material constraints are necessary as is information about the environment where the antenna is to be placed. Gain, beamwidth, feed impedance, RF power levels and so on. Cost considerations are also a big part of the design. Is the horn to be used as a feed or will it operate by itself? The list goes on.