Waveguide horn antennas, also known as horn antennas, are composed of a uniform waveguide and a flared horn with increasing cross-sectional area. They are one of the most commonly used microwave antennas and are generally used as radiators. The advantages of waveguide horn antennas are their broad operating frequency bandwidth, but their disadvantages are their large size and less directional compared to parabolic antennas of the same aperture.
Waveguide horn antennas are aperture antennas whose internal metal walls gradually transition to ensure impedance matching between the waveguide port and free space. This can adjust the aperture and axial length of the waveguide horn antenna to obtain the desired gain and directivity for a given application.
Waveguide antennas can be pyramid-shaped (rectangular) horns, conical horns, sector horns, horn lenses, corrugated feed horns, probes, or omnidirectional horns.
In addition, waveguide horn antennas can be designed with metal ridges or dielectric structures that change the impedance or bandwidth characteristics of the horn antenna.
Generally speaking, waveguide horn antennas exhibit high gain and highly directional beam characteristics, making them very suitable for microwave/millimeter wave testing, sensing (radar), and satellite communications (often paired with parabolic antennas).
In the waveguide horn antenna, the planar electromagnetic wave propagated by the waveguide gradually transforms into a curved wavefront propagated in free space. In rectangular antennas, the waveguide mode is usually TE10 or TE01 with a curved wavefront.
Sector horn antennas emit cylindrical wavefronts, and conical horn antennas emit spherical wavefronts. The expansion of the horn affects the gain, beam width, and directivity of the antenna response. The wider the expansion, the wider the beam width and the lower the directivity and gain.
Pyramid/rectangular horn antennas often suffer from considerable side lobes due to phase errors associated with horn transitions. Nonlinear expansions of the horn (soft horns), such as corrugations, dielectric wall linings, or strips perpendicular to the direction of electromagnetic wave propagation, are sometimes used to correct this.
Various waveguide horn antennas exhibit different bandwidth characteristics, but "simple" horn structures tend to have narrow-bandwidth operations due to impedance-matching transitions. To improve this bandwidth, ridges or dielectric lenses/inserts can be added to allow for greater bandwidth impedance matching and greater utilization of the waveguide band. Dielectric lenses can also be used to convert the curved wavefront at the output end of the standard horn antenna into a planar wavefront, often referred to as a horn lens antenna.
Given the high gain, directivity, and power handling of waveguide horns, these antennas are often used for testing other antennas, characterizing materials in free space, testing electromagnetic compatibility/electromagnetic interference (EMC/EMI), radar, and other applications.