The horn antenna is used in the transmission and reception of RF microwave signals, and the antenna is normally used in conjunction with waveguide feeds. A horn antenna form of antenna that consists of a flared waveguide which is shaped like a horn and it has the effect that it enables a transition between the waveguide and free space and it also directs radio waves in a beam.

The horn antenna may be considered as an RF transformer or impedance match between the waveguide feeder and free space which has an impedance of 377 ohms. By having a tapered or having a flared end to the waveguide the horn antenna is formed and this enables the impedance to be matched. Although the waveguide will radiate without a horn antenna, this provides a far more efficient match.

In addition to the improved match provided by the horn antenna, it also helps suppress signals travelling via unwanted modes in the waveguide from being radiated. However the main advantage of the horn antenna is that it provides a significant level of directivity and gain. For greater levels of gain the horn antenna should have a large aperture. Also to achieve the maximum gain for a given aperture size, the taper should be long so that the phase of the wave-front is as nearly constant as possible across the aperture. However there comes a point where to provide even small increases in gain, the increase in length becomes too large to make it sensible. Thus gain levels are a balance between aperture size and length. However gain levels for a horn antenna may be up to 20 dB in some instances. Horn antenna types

There are several types of horn antenna:

• Pyramid horn antenna: As the name suggests, the pyramid horn antenna takes on a rectangular shape - the cross section through the antenna is rectangular, as is the end of the antenna. It is normally used with rectangular waveguide.
• Sectorial horn antenna: This form of horn antenna is one in which only one pair of sides flared whist the other remains parallel. This form of configuration produces a fan-shaped beam, which is narrow in the plane of the flared sides, but wide in the plane of the narrow sides.

1. E-plane horn antenna: This form of antenna is one that is flared in the direction of the electric or E-field in the waveguide.

               2. H plan horn antenna: This form of antenna is one that is flared in the direction of the electric or H-field in the waveguide.

• Conical horn antenna: Again, as the name indicates, the conical horn antenna has a circular cross section and end into it. It is normally used with circular waveguide and is seen less frequently than the rectangular version.
• Exponential horn antenna: This form of horn antenna is also called a scalar horn antenna and it is one that has curved sides. The separation of the sides increases as an exponential function of length. The antenna can come as either a pyramidal or conical cross sections. The advantage of exponential horns is that they have a minimum level of internal reflections, and almost constant impedance and other characteristics over a wide frequency range. They are used in applications requiring high performance, such as feed horns for communication satellite antennas and radio telescopes.
• Corrugated horn: The corrugated horn antenna has parallel slots or grooves along the inside surface of the horn, transverse to the axis. These corrugations are small when compared to the wavelength. Corrugated horns have several advantages including a wider bandwidth, and smaller side-lobes that other types. The corrugated horn provides a pattern that is nearly symmetrical, with the E and H plane beam-widths being nearly the same. As a result corrugated horn antennas are widely used as feed horns for satellite dishes and radio telescopes.

Horn antenna applications

Horn antennas are used in many areas, not only because they are convenient, but because they possess a number of features that make them ideal in many applications. Some of the main applications for horn microwave antennas are: - Gain standards: The horn antenna is a very convenient form of microwave antenna that can be used. As it has a very wide bandwidth, its performance varies little over a wide frequency range. - Feeds for parabolic reflector antennas: Parabolic reflectors need another antenna to "illuminate" the parabolic reflector itself. The horn antenna often known as a feed horn in this application, possesses sufficient directivity to illuminate the reflector sufficiently evenly without too much spillage over the edge of the dish. The use of the horn antenna also minimizes the spurious responses of the parabolic reflector antenna to signals that are not in the main lobe. - Short range radar systems: One particular use of horn antennas is for use in speed enforcement cameras.

The horn antenna is a particularly useful form of antenna for use with RF microwave applications and waveguide feeder. Although it is not used below RF microwave frequencies because waveguides are not used at low frequencies as a result of the sizes needed, the horn antenna is nevertheless a very useful form of RF antenna design for use at high frequencies.