A short backfire antenna (or short back-fire, SBA, SBF or SBFA) is a type of a directional antenna, characterized by high gain, relatively small size, and narrow band. It has a shape of a disc with a straight edge, with a vertical pillar with a

dipole In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: * An electric dipole moment, electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple ...

acting as the

driven element In an antenna array made of multiple Electrical conductivity, conductive elements (typically metal rods), a driven element or active element (also called driven radiator or active radiator) is electrically connected to the radio receiver, rec ...

in roughly the middle and a conductive disc at the top acting as a sub-reflector. The bottom disc has the diameter of two wavelengths, and its collar (edge) is quarter the wavelength tall. The center pillar consists of two coaxial tubes (their diameter has to be carefully chosen to give the desired impedance), with a quarter-wavelength slot cut into the outer tube. This structure behaves like a

resonant cavity A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a resona ...

, resulting in a substantial gain in small space. Short backfire antennas are used in some

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

s, and in high-frequency (short-wavelength) communication equipment (often for communication with satellites) on ships and other applications where rugged construction is an advantage. They are also used for

wireless LAN A wireless LAN (WLAN) is a wireless computer network that links two or more devices using wireless communication to form a local area network (LAN) within a limited area such as a home, school, computer laboratory, campus, or office building ...

s. The SBF antenna was invented by Dr. Hermann W. Ehrenspeck of Air Force Cambridge Research Labs based at

Hanscom Air Force Base Hanscom Air Force Base (AFB) is a United States Air Force base located predominantly within Bedford, Massachusetts, with portions extending into the adjoining towns of Lincoln, Concord and Lexington. The facility is adjacent to Hanscom Field ...

in Bedford, MA and was used for, among other purposes, to provide Tactical Satellite Communications for U.S. Army ground forces due to the SBF's portability and gain. The bandwidth of the antenna can be increased by using a conical main reflector instead of a flat one. Archery target antenna is an evolution of the short-backfire antenna. The name is derived from its appearance. Its base diameter is 5 wavelengths (5 λ), the rim height is 0.7 λ, the diameter of the small center reflector is 0.7 λ, its height above the base is 0.7 λ as well, the height of the dipole above the base is 0.35 λ, there is one more reflector - an annular reflector - at the same height as the center reflector, with inner diameter 2.2 λ and outer diameter 3.7 λ

Short backfire antennas are able to achieve high aperture efficiencies, at right cavity size even beyond 100%.

References

{{Reflist Radio frequency antenna types Antennas (radio)

See also

References