Microwave Horn Antenna

The horn antenna is used particularly at microwave frequencies to provide an effective, yet straightforward antenna for many applications.

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Horn Antenna Includes:
Horn antenna basics Horn antenna theory & equations

The horn antenna has a distinctive shape and is unlike many other forms of antenna and is used at microwave frequencies.

The horn antenna can be considered to be a waveguide that has been widened out in the form of a horn. As a result it finds many applications in areas where waveguides are used.

Shaped like a horn and this antenna forms a smooth transition between the waveguide and free space whilst also directing the radio waves in a beam.

A typical horn antenna — Typical microwave horn antenna
showing coaxial feed into a waveguide transition

Basic horn antenna concept

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.

Diagram of a horn antenna often used with waveguide feeder to provide a convenient microwave antenna showing the horn forming a pyramid at the end of the waveguide — Microwave horn antenna

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.

When the horn needs to be used with coax, a small section of waveguide is required in which a waveguide to coax transition is located.

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.
Sectoral 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.
- 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.
- 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 ito 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 advantages

The microwave horn antenna is relatively straightforward in its appearance and as a result, its advantages may be overlooked.

Nevertheless the horn antenna is very useful as a result of the properties it possesses.

Wide bandwidth: The horn antenna possesses no resonant elements and therefore it is able to operate over a wide bandwidth.
Easy interface to waveguide: By the very nature of their shape, these antennas are very easy to interface to waveguide, although they can also be designed with a transition so that standard coaxial feeder can also be used.
Simple construction: The horn antenna consists simply of a flared horn. As a result they are relatively easy to construct.

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:

Feeds for parabolic reflector antennas: Horn antenna provide an excellent method for feeding parabolic reflector antennas. Parabolic reflectors need another antenna to "illuminated" the reflector element of the antenna and the horn antenna provides a very convenient method of achieving this. In this application the horn antenna is known as a ‘feed horn’ in and it can be designed to provide 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. Here the horn antenna provides a useful amount of directivity to prevent other reflections interfering with the required response.
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. The horn antenna may often be used in this type of application for EMC and other similar measurements.

The horn antenna is only seen at microwave frequencies because it would be far to large for use at lower frequencies. Where it is used, the horn antenna provides an effective for of antenna that is relatively straightforward to implement and manufacture.