- an overview of the basics of the horn antenna used in RF microwave applications.
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. The horn antenna gains its name from its appearance. The waveguide can be considered to open out or to be flared, launching the signal towards the receiving antenna.
Horn antennas are often used as gain standards, and as feeds for parabolic or 'dish' antennas, as well as being used as RF antennas in their own right. One particular use of horn antennas themselves is for short range radar systems, such as those used for automotive speed enforcement.
When used as part of a parabolic reflector, the horn is orientated towards the reflector surface, and is able to give a reasonably even illumination of the surface without allowing radiation to miss the reflector. In this way it is able to maximise the efficiency of the overall antenna. 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.
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.
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 two basic types of horn antenna: pyramid and conical. The pyramid ones, as the name suggests are rectangular whereas the corrugated ones are usually circular. The corrugated horn provides a pattern that is nearly symmetrical, with the E and H plane beamwidths being nearly the same. Additionally it is possible to control the side lobes better with a conical or corrugated horn antenna.
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.
By Ian Poole
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