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Asector antenna is a type ofdirectionalmicrowaveantenna with asector-shapedradiation pattern. The word "sector" is used in the geometric sense; some portion of the circumference of a circle measured in degrees of arc. 60°, 90° and 120° designs are typical, often with a few degrees 'extra' to ensure overlap and mounted in multiples when wider or full-circle coverage is required (see photos below). The largest use of these antennas is as antennas forcell phonebase-station sites. They are also used for other types ofmobile communications, for example inWi-Fi networks. They are used for limited-range distances of around 4 to 5 km.
A typical sector antenna is depicted in the figure on the right. At the bottom, there areRF connectors forcoaxial cable (feedline), and adjustment mechanisms. For its outdoor placement, the mainreflector screen is produced fromaluminum, and all internal parts are housed into afiberglassradome enclosure to keep its operation stable regardless of weather conditions.
Grounding is very important for an outdoor antenna so all metal parts areDC-grounded.
The antenna's long narrow form gives it a fan-shapedradiation pattern, wide in the horizontal direction and relatively narrow in the vertical direction. According to the radiation patterns depicted, typical antennas used in a three-sectorbase station have 66° of horizontalbeamwidth. This means that the signal strength at the ±33° directions is half (3 dB down) from its peak value at the center. At the ±60° directions, it is suggested to be a border of a sector and antenna gain is negligible there.
Vertical beamwidth is not wider than 15°, meaning 7.5° in each direction. Unlike antennas forcommercial broadcasting - AM, FM and television for example - which must achieveline-of-sight over many miles or kilometers, there is usually a downwardbeam tilt or downtilt so that the base station can more effectively cover its immediate area and not causeRF interference to distant cells.
Thecoverage area, which is determined by the projection of the radiation pattern on the ground, can be adjusted by changing the downtilt of the pattern. In some models this is done mechanically by manually adjusting the tilt of the antenna with an adjustable mounting bracket. In more recent sector antennas the pattern can be electronically tilted, by adjustablephase shifters in the feed of the individual dipole elements. These are adjusted by aremote control circuit from the ground, eliminating the need for a technician to climb the antenna tower.
To increase or widen the coverage area, and thus the number of served clients, several sector antennas are installed on the same supporting structure, e.g.tower or mast.[citation needed]
Such a construction is often called asectorized antenna, though sometimes for brevity "sector antenna" is used as well. It has several angularly-separated sector antennas as shown on the figures at right.
Once the antenna unit is attached to a supporting structure, it has to be positioned. Positioning means not only setting a correct direction orazimuth, but setting a correct downtilt as well. By restricting emitted energy to a sub-circular arc and narrow vertical coverage the design makes efficient use of relatively low power transmitter equipment. Though absolute range is limited, this configuration allows for gooddata rates (digital information transfer measured in bits/second, sometimes given as total minus error-correction overhead), and good signal consistency within the coverage area.
Prior to positioning, grounding and lightning protection are required. As seen in the pictures, all supporting constructions havelightning rods.
A well-chosen downtilt setting strategy can lower the overall interference in the network. A too-aggressive downtilting strategy will however lead to an overall loss of coverage due to cells not overlapping. Downtilting can be used to solve specific problems, for example local interference problems or cells that are too large. Electrical tilting slightly reduces beam width.
In a picture on the right, there are two sector antennas with different mechanical downtilts. Note that a more vertical antenna is less visible than a mechanically tilted one - the use of purely electrical tilt with no mechanical tilt is therefore an attractive choice for aesthetic reasons which are very important for operators seeking acceptance of integrated antennas in visible locations.