Short backfire antenna

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Short backfire antenna

A short backfire antenna (abbreviated SBA , SBF , SBFA and English short back-fire ) is a directional antenna for receiving or transmitting electromagnetic waves in the range from approx. 100  MHz to approx. 3 GHz.

Structure and description

The short backfire antenna is the shortest possible backfire antenna type . The antenna is approx. 0.5 λ long. With the short backfire antenna, gains of up to approx. 15 dBi can be achieved, with a front / back ratio of approx. 30 dB. The sidelobe attenuation reaches values ​​of greater than 20 dB.

The antenna consists of two flat circular reflectors. The main reflector has a diameter of 2λ. The auxiliary reflector has a diameter of 0.5λ. The short backfire antenna consists of only three components, the main reflector, the fed element and the auxiliary reflector. The two reflectors are mostly circular, but they can also be made in the shape of a square, rectangle or a polygon (hexagon, octagon, etc.). A relatively wide collar of the main reflector increases the back attenuation in particular. The two reflectors can consist of a close-meshed grid (mesh size <0.1λ) or of parallel rods. The main reflector can also consist of a metal disc. A circular sheet metal disc is recommended for the auxiliary reflector (which can also have small cutouts). There are no binding regulations for the execution of the supplied exciter element. The antenna can have a horizontally or vertically polarized exciter. The fed element can also be a circularly polarized turnstile antenna. With horizontal polarization of the exciter, the high antenna gain results primarily from the bundling in the vertical plane. The radiation properties of the short backfire antenna are similar to those of a group antenna consisting of 2 stacked Yagi-Uda antennas .

The short-backfire antenna can be given broadband properties if it is provided with a corresponding broadband excitation element. For this purpose, the antenna system should be dimensioned for the highest possible working frequency. The maximum gain is then achieved at the highest working frequency. As the frequency falls - approximately proportional to the frequency - the antenna gain slowly decreases. At low frequencies - when the auxiliary reflector has become small compared to λ / 2 - the backfire effect disappears. The short backfire antenna then becomes a simple reflector antenna - provided that the frequency range of the pathogen is sufficient. The interconnection of short backfire antennas in groups - which can also be located in front of a common main reflector of appropriate size - is possible.

Varia

The short backfire antenna is particularly suitable as a UHF television antenna. The short backfire principle is also ideal for circular polarization. The circular polarization occurs when the fed dipole is replaced by a turnstile antenna. For example, in order to be able to operate a Lang Yagi Uda antenna with circular polarization, the total number of elements would have to be doubled. With a short backfire antenna - apart from replacing the fed dipole with a turnstile exciter - no additional work is required.

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