Backfire antenna
A backfire antenna (also abbreviated to BA , BF , BFA , LBA , LBF , LBFA , back-fire and long back-fire ) is a directional antenna which is used as a transmitting and receiving antenna in the frequency range from approx. 100 MHz to approx. 3 GHz. It belongs to the group of Yagi-Uda antennas and was first described by Hermann W. Ehrenspeck in the late 1950s.
Structure and description
The backfire antenna is a combination of a Yagi-Uda antenna with a reflector wall. From the Backfire antenna, there are two versions: In addition to the described long Backfire antenna, English Long Backfire which multiple wavelengths is λ long, the short backfire antenna . The antenna gain is a function of the large reflector area and the length of the antenna. With the "Long Backfire" gains of up to approx. 25 dBi can be achieved with a standing wave ratio of approx. 30 dB.
The mode of operation of the antenna can easily be explained for the transmission case: The radiation emanating from the fed element, with the support of the auxiliary reflector, is guided via the waveguide system of the directors to the large-area backfire reflection wall. After reflection, the radiation therefore passes through the Yagi-Uda structure a second time in the opposite direction and is emitted into free space as bundled radiation.
A backfire antenna has roughly the same characteristics as a Lang Yagi Uda antenna with twice the length. The Yagi-Uda section, for example, has a length of approximately 1.5 λ; the antenna therefore corresponds to a Lang-Yagi-Uda of approx. 3 λ length and a double number of elements with regard to its characteristic radiation properties.
With a Yagi-Uda antenna, doubling the antenna length and the number of elements increases the antenna gain by approx. 2.5 dB. The relatively large backfire wall as a reflective surface causes a gain increase of approx. 4 dB to 6 dB, based on a Yagi-Uda antenna of the same length without a backfire wall.
The size of the reflective surface significantly influences the antenna gain. The general rule here is: the longer the Yagi-Uda structure used, the larger the backfire reflection wall.
A Yagi-Uda antenna that is optimally dimensioned without a reflector wall changes its resonance properties significantly when it is connected to the Bachfire reflective surface. During the development of the short backfire antenna , it became clear that large maximum gains can be achieved with relatively short antenna lengths.
In order to get the maximum antenna gain again, the element spacing can be retained; however, all element lengths must be changed. As an approximation rule, the following applies: The fed element must be lengthened, and the reflectors must also be enlarged. The directors, however, must be shortened.
Backfire antennas for receiving TV bands IV / V have been developed by the industry.
literature
- Rothammels Antennenbuch , DARC Verlag GmbH, Baunatal 2002, ISBN 3-88692-033-X , p. 632f.