To improve the radiation efficiency and directivity of the beam, the waveguide should be provided with an extended aperture so as to make the abrupt discontinuity of the wave into a gradual transformation, so that all the energy in the forward direction gets radiated. This can be termed as Flaring. Flaring is done to provide greater directivity and narrower beam width. The types of horn antennas depend on how they are flared.
This type of horn antenna, flares out in only in one direction. Flaring in the direction of Electric vector produces the sectorial E-plane horn. As well, flaring in the direction of a Magnetic vector produces the sectorial H-plane horn.
This type of horn antenna has flared on both sides. If flaring is done on both the E & H walls of a rectangular waveguide, then pyramidal horn antenna is produced. This antenna has the shape of a truncated pyramid.
When the walls of a circular waveguide are flared, such an antenna can be termed as a conical horn. This is a logical termination of a circular waveguide.
These horn antennas may also be combined with parabolic reflector antennas to form a special type of horn antennas. They are
In Cass-horn antenna, radio waves are collected by the large bottom surface which is parabolically curved and reflected upward at 45˚angle. After hitting the top surface, they are reflected the focal point. The gain and beam width of these are just like parabolic reflectors.
In a hog-horn antenna, a parabolic cylinder is joined to a pyramidal horn, where the beam reaches the apex of the horn. It forms a low-noise microwave antenna. The main advantage of a hog-horn antenna is that it’s receiving point doesn’t move, though the antenna is rotated about its axis.