Horn antennas are widely used in high power microwave systems, mainly due to their high power handling properties. Different variations of the traditional horn antenna geometry have been introduced in the past years to improve the radiation characteristics. Some of these techniques are used to alter the boundary conditions on the walls of the horn antennas allowing for the propagation of hybrid modes (hybrid mode horns). As an example, corrugations can be used to create hard and soft horn antennas with reduced side lobes, increased radiation efficiency and low cross polarization levels. Similar to corrugations, metamaterials can be added to horn antennas to generate hybrid modes.