Short backfire antenna
Encyclopedia
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
acting as the driven element
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, resulting in a substantial gain in small space.
Short backfire antennas are used in some satellite
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
s. The SBF antenna was invented by Dr. Hermann W. Ehrenspeck of Air Force Cambridge Research Labs based at Hanscom Air Force Base
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. http://www.mwjournal.com/default.asp?journalid=1&func=articles&page=0505m28&year=2005&month=5&rsno=5
It has a shape of a disc with a straight edge, with a vertical pillar with a dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
acting as the driven element
Driven element
In a multielement antenna array , the driven element or active element is the element in the antenna which is electrically connected to the receiver or transmitter. In a transmitting antenna it is driven or excited by the RF current from the transmitter, and is the source of the radio waves...
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
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
), with a quarter-wavelength slot cut into the outer tube
This structure behaves like a resonant cavity, resulting in a substantial gain in small space.
Short backfire antennas are used in some satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
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
Wireless LAN
A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...
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
Hanscom Air Force Base is a United States Air Force base located approximately south-southwest of Bedford, Massachusetts. The facility is a joint use civil airport/military base with Hanscom Field which provides general aviation and charter service.The host unit at Hanscom is the non-flying...
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. http://www.mwjournal.com/default.asp?journalid=1&func=articles&page=0505m28&year=2005&month=5&rsno=5