The phased array is a directive antenna mode up of individual radiating antenna, or elements, which generate a radiation pattern whose shape and direction is determined by the relative phases and aptitudes of the currents at the individual elements.By properly varying the relative phases, it is possible to steer the direction of the radiation.The radiating elements might be dipoles, open-ended waveguides,slots cut in wave guide, or any other type of antenna.The inherent flexibility offered by the phased-array antenna in steering the beam by means of electronic control is what has made it if interest for radar.It has been considered in those radar applications where it is necessary to shift the beam rapidly from one position in space to another, or where it is required to obtain information a bout many targets at a flexible,rapid date rate.The full potential of a phased-array antenna requires the use of a computer that can determine in real time, on the basis of the actual operational situation,
The concept of directive radiation from fixed (non steered) phased-array antennas was known during world war 1. The first use of the phased-array antenna in commercial broad casting transmission was in the early thirties and the first large steered directive array for the reception of transatlantic short-wave communication was developed and installed by the Bell Telephone Laboratories in the late thirties, In world war 2, the United States, Great Britain,and Germany all used radar with fixed phased-array antenna in which the beam wad scanned by mechanically actuated phase shifters. In the United States,this was an azimuth scanning S-band fire control radar, the Mark 8, that was widely used on cruisers and battleship's, and the AN/APQ-7 ( Eagle) high-resolution navigation and bombing radar at X band that scanned a 0.5 (deg) fan beam over a 60(deg) sector in 1 1/3 seconds. The British used the phased array in two height-finder radars, one at VHF and the other at S band.The Germans employed VHF radars with fixed planer phased arrays in significant numbers. One of these called the Mammut, was 100 ft wide and 36 ft high and scanned a 10(deg) beam over a 120(deg) sector
A major advance in phased-array technology was made in the early 1950"s with the replacement of mechanically actuated phase shifters by electronic phase shifters. Frequency applied.in one angular coordinate was the first successful electronic scanning technirqe to be applied.In terms of numbers of operational radars, frequency scanning has probably seen more application than any other electronic scanning method.The first major electronically scanned phased arrays that performed beam steering without frequency scan employed the Huggins phase shiftier which, in a sense, used the principle of frequency scan without the necessity of changing the radiated frequency.The introduction of digitally switched phase shifters employing either ferrites or diodes in the early 1960s made a significant improvement in the practicality of phased arrays that could be electronically steered in two orthogonal angular coordinates
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