Sep 9, 2016
The Three Broad Types of Antennas
There are several different types of antennas in three broad categories: omni-directional, directional, and semi-directional. Following are the types of Directional Antenna: Sector antenna
. Sector antennas provide a pie-shaped (sector) radiation pattern and are usually installed in what is known as a sectorized array. Beam width for a sector antenna can be between 60 to 180 degrees, with 120 degrees being typical. In a sectorized array, antennas are mounted back-to-back to provide full 360-degree coverage. Sector antennas are used extensively for cellular communication. Yagi antenna A commonly used directional antenna is the Yagi-Uda Array, usually just called a Yagi. A Yagi antenna uses several elements to form a directional array.
; elements placed immediately in front of and behind the driven element re-radiate RF energy in phase and out of phase, enhancing and retarding the signal, respectively. The elements are called parasitic elements; the element behind the driven element is called the reflector, while the elements in front of the driven element are called directors. Yagi antennas have beamwidths in the range of 30 to 80 degrees and can provide well in excess of 10 dBi passive gain. Parabolic or dish antenna Parabolic, or dish, antennas are the most familiar type of directional antenna. A parabola is a symmetric curve; a parabolic reflector is a surface that describes that curve throughout a 360-degree rotation��"a dish or, to use the technical term, a paraboloid. A parabolic reflector has a high degree of directivity and has the ability to focus RF energy into a beam, much like a flashlight. Parabolic antennas have a very narrow beamwidth, usually not exceeding 25 degrees. Gain is dependent on diameter and frequency; at 2.4 GHz, a 1 meter dish will provide about 26 dBi gain, while a 10 meter antenna will provide 46 dBi gain at the same frequency. The antenna is “fed” by either a half wave dipole antenna or a feed horn. Parabolic antennas are used for long distance communication links between buildings or over large geographic areas. Very large parabolic antennas are used for radio astronomy and can provide gain of 10 million or about 70 dBi. Grid antenna A variation of the dish is the grid antenna. Given that a parabolic reflector will present a large solid surface to the wind, it follows that high or even moderate wind conditions will cause the dish to move out of alignment or deform. To prevent this from happening, the reflector is perforated into a grid. The spacing of the grid elements is frequency dependent; it is inversely proportional to the frequency. Gain and beam width are similar to the parabolic antenna.
Posted at 02:40 am by samsolong
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