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An antenna array is a set of individual antennas used for transmitting and/or receiving radio waves, connected together in such a way that their individual currents are in a specified amplitude and phase relationship. The interactions of the different phases enhances the signal in one desired direction at the expense of other directions. This allows the array to act as a single antenna, generally with improved directional characteristics (thus higher antenna gain) than would be obtained from the individual elements. The resulting array in fact is often referred to and treated as "an antenna," particularly when the elements are in rigid arrangement with respect to each other, and when the ratio of currents (and their phase relationships) are fixed. On the other hand, a steerable array may be fixed physically but has electronic control over the relationship between those currents, allowing for adjustment of the antenna's directionality without requiring physical motion. The concept is widely used and has various names; one relatively common synonym is directional array. When used with a reflector to further improve the directionality, the result is a reflective array antenna. If each of the individual antennas within the array can be separately controlled, the result is a phased array. Purposes include the production of a null to avoid co-channel interference with another transmitter. ==Classifications== An antenna array may be classified as ''parasitic'' or ''driven''.〔 Navy Electricity and Training Series. Module 11 - Microwave Principles. * *〕 The best known parasitic array is the Yagi-Uda antenna, consisting of several elements but only one of which has an electrical connection to the transmitter or receiver. The other elements are electromagnetically coupled to that element (and to each other) through proximity, and are tuned so that their currents will be in the appropriate phases to enhance the directionality of the resulting array. A driven array implies that all of the elements have an electrical connection to the transmitter or receiver, through circuitry that tailors their respective currents to the same end. One common example is the log-periodic dipole array frequently used as a rooftop TV antenna. Driven arrays, in which all the radiating elements are connected to the energy source, have smaller losses than parasitic arrays. Driven arrays often (but not always) consist of elements which are identical; frequently these are just half-wave dipole or quarter-wave monopole (vertical) elements. The resulting array inherits the characteristics of that basic element, but the resulting array behaviour is determined more by the geometrical arrangement of the elements and the phases (and sometimes amplitudes) which each is assigned. Typically there will be a number of identical elements equispaced along one direction. In that case, an ''end-fire array'' is one in which the intended directionality is ''in'' the direction of the array. A ''broadside array'' has its intended directionality ''at right angles'' to the array direction. Of course there are also more complex arrays possible in which these classifications do not apply. Sometimes a broadside array consists of identical elements arranged vertically. In this case, the directional pattern of the resulting antenna in the horizontal plane may be unchanged from that of a single such element. However, by adding those additional elements, the antenna pattern in the ''vertical'' plane is altered in order to supply a greater amount of power in horizontal directions (in order to communicate with terrestrial locations) at the expense of directions aimed more toward the sky or ground which are not useful. Consequently, the gain of the antenna is increased without sacrificing directionality towards any intended stations. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Antenna array (electromagnetic)」の詳細全文を読む スポンサード リンク
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