Sector antenna
A sector antenna is a kind of directional antenna with a sector-shaped radiation pattern. In mobile communications, these antennas are typically installed in base station sites for point-to-multipoint connections.
Structure, Materials and Radiation Pattern
Typical sector antenna is depicted on a figure below. In the bottom, there are connectors for RF feeders and adjustment mechanisms. For its outdoor placement, main reflector screen is produced from aluminum and all internal parts are housed into a fiberglass enclosure to keep its stable function regardless to weather conditions.
Grounding is very important for an outdoor antenna so all metal parts are DC grounded.
The directivity is basically achieved by antenna's narrow shape. According to radiation patterns depicted, typical antenna used in 3 sector base station has 66° of horizontal beam width. This means that the maximum gain is achieved in 0° and its value is slightly low in -33°/+33° direction. At 60° direction, it is suggested to be a border of a sector and antenna gain is negligible there.
Vertical tilt or vertical beamwidth, sometimes called downtilt, is not wider than 15°, meaning 7.5° in each direction.
The coverage area which is equal to the square of sector's projection to the ground can be adjusted by changing electrical or mechanical downtilts. Electical tilt is set by using a special control unit which usually is built into the antenna case, though different remote control devices are widely produced. Mechanical downtilt is set manually by adjusting an antenna fastener.
Practical Use
To increase a coverage area and thus, a number of served subscribers, several sector antennas are installed on the same supporting structure, i.e. tower or mast.
Such a construction is often called a sectorized antenna, though sometimes for brevity sector antenna is used as well. It has several angularly separated sector antennas as shown on a figure.
Once the antenna unit is attached to a supporting structure, it has to be positioned. Positioning means not only setting a correct direction or azimuth but setting a correct downtilt as well.
Prior to positioning, grounding and lightning protection are performed. As it seen from the pictures, all supporting constructions have lightning rods.
A well-chosen downtilt setting strategy can lower the overall interference in the network. A too aggressive down tilting strategy will however lead to an overall loss of coverage due to sectors overlapping. In addition to a general down tilt strategy, applied in all cells, downtilt can be used to solve specific problems, for example local interference problems or cells that are too large.
In a picture on the right, there are two sector antennas with different mechanical downtilts.
Vendors
All major telecommunication equipment manufactures have their own antennas, including:
and many others. There are lot of "antennas only" vendors, the major of them are:
and others.