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Since 1888 when dish antennas were invented, their use has steadily been getting more widespread. In today's technological age, applications for these antennas keep growing. In the modern landscape, the parabolic disk antennas can be seen in most urban settings. Their use has been popularized by the advent of satellite dish television. These antennas have other important applications.
Antennas which use parabolic reflectors or curved surfaces with cross-sectional shapes to direct radio waves are called parabolic antennas. These antennas are most commonly designed into dish-like shapes. Because these discs can direct radio waves in narrow beams or receive the waves only in one single direction, their power is high. Their mode of functioning resembles flash lights or search lights.
In point to point communication, these antennas are used as high gain antennas. Their power makes them effective for many applications. They can be used for microwave relay links. This means that they can carry or receive radio signals from one city to the next.
These antennas can be used as radar antennas. They can transmit radio waves in narrow beams to locate large objects such as airplanes, ships, and guided missiles. For communication purposes, they are useful for WAN/WLAN links for data, for spacecraft and satellite communication.
Parabolic disk antennas have a parabolic metal reflector and small feed antennas. The feed antennas are located at the focus of the reflectors pointing backwards. Reflectors are usually metallic surfaces formed into parabolic shapes. Radio frequency current from transmitters is sent through transmission line cables to the feed antenna. These are then converted into radio waves. Radio waves are emitted back to the dish through the feed antenna and are reflected off the dish into parallel beams. Incoming waves bounce off receiving antenna and are focused onto the feed antenna. This converts them into electric current that is then relayed through a transmission line to a radio receiver.
Parabolic dish antennas can be categorized by their shapes
Cylindrical antennas- In these antennas the reflectors are curved in a single direction while the other is flat. The curved reflector ends are sometimes capped by flat plates. This prevents radiation from passing out from these ends.
Shape beamed parabolic antennas-These modern reflector antennas are designed to produce beams of the desired shape. They therefore produce wider beams rather that the pencil or narrow beams.
Shaped reflectors- Parabolic reflectors can be designed into shapes that are non-cylindrical. These designs are normally used in radar antennas.
Shrouded dish-This design has a metal shield that is attached to the cylindrical disc rim. The shield protects the antenna from radiation that is outside the beam axis. This is ideal for preventing interference from terrestrial radio waves.
Parabolic antennas can also be categorized according to their feeds. Some common examples include:
Axial or front feed-This feed type is the most common. Its feed antenna is located at the front of the disk.
Offset or off-axis feed-In this antenna design the reflector and feed are located on the same side of the dish. This moves the feed structure from the path of the beam.
Cassegrain. In this design the feed is situated behind or on top of the dish. The radio waves from feed are reflected back to the dish from the reflector.
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Source by Charles O. Momo
