Hat Feed Antenna Capable of Sum and Difference Pattern Synthesis
  Potentially Separate waves could propagate independently with arbitrary amplitude and phase, if one longitudinal septum is inserted in the H-Plane of the circular waveguide. The presented paper has implemented this idea in order to excite hat feed antenna simultaneously with two in-phase and out of phase signals and leads to sum and difference patterns. 
Morteza Shahpari, Gordana K. Felic
physics.optics