what is horn antenna

The horn antenna is a transition between a waveguide and free space. A rectangular
waveguide feed is used to connect to a rectangular waveguide horn, and a circular
waveguide feed is for the circular waveguide horn. The horn antenna is commonly
used as a feed to a parabolic dish antenna, a gain standard for antenna gain
measurements, and as compact medium-gain antennas for various systems. Its gain
can be calculated to within 0.1 dB accuracy from its known dimensions and is
therefore used as a gain standard in antenna measurements.