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A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish. The main advantage of a parabolic antenna is that it has high directivity.
Correspondingly, the dimensions of a symmetrical paraboloidal dish are related by the equation: =, where is the focal length, is the depth of the dish (measured along the axis of symmetry from the vertex to the plane of the rim), and is the radius of the dish from the center. All units used for the radius, focal point and depth must be the same.
The focal ratio (f-number, the ratio of the focal length to the dish diameter) of typical parabolic antennas is 0.25–0.8, compared to 3–8 for parabolic mirrors used in optical systems such as telescopes. In a front-fed antenna, a "flatter" parabolic dish with a long focal length would require an impractically elaborate support structure to ...
It consists of a horn antenna with a reflector mounted in the mouth of the horn at a 45 degree angle so the radiated beam is at right angles to the horn axis. The reflector is a segment of a parabolic reflector, and the focus of the reflector is at the apex of the horn, so the device is equivalent to a parabolic antenna fed off-axis. [22]
This distance provides the limit between the near and far field. The parameter D corresponds to the physical length of an antenna, or the diameter of a reflector ("dish") antenna. Having an antenna electromagnetically longer than one-half the dominated wavelength emitted considerably extends the near-field effects, especially that of focused ...
The parabolic shape of a dish reflects the signal to the dish's focal point. Mounted on brackets at the dish's focal point is a device called a feedhorn. This feedhorn is essentially the front-end of a waveguide that gathers the signals at or near the focal point and 'conducts' them to a low-noise block downconverter or LNB.
As in a front-fed parabolic dish, the feed is located at the focal point of the reflector, but the reflector is an asymmetric segment of a paraboloid, so the focus is located to the side. The purpose of this design is to move the feed antenna and its supports out of the path of the incoming radio waves.
The central focal area would be rectangular, but a secondary rectangular-parabolic mirror would gather the light to a focal point. Otherwise the optics are similar to other optical telescopes. The light gathering power of a Rice telescope is equivalent to approximately the width times the diameter of the ring, minus some fraction based on ...