Beam direction of the difference pattern in a monopulse phased array antenna

The beam direction of a monopulse phased array antenna can be changed rapid ly by controlling the phase shifters, which is useful in detecting the dire ction of arrival of radio waves. In monopulse tracking, the null point of t he difference pattern is used as the reference for direction determination. However, errors of phase setting, including the quantization error of the phase shifters, give rise to the problem of null shift, that is, the null p oint shifts from the set direction. This study examines the relation betwee n the phase setting error and the null shift from the statistical point of view and expresses the relation between the two in a simple formula. In the general evaluation function, the position of the element antenna and the e xcitation amplitude are used as the parameters. Then, an expression is deri ved by approximating the amplitude distribution over the antenna aperture a s a quadratic function, with the result that the evaluation function can be derived without defining the element position. The effectiveness of these expressions is verified by numerical calculation examples. In addition, the minimum change of the beam direction, which determines the angular measure ment accuracy of the monopulse phased array antenna, depends on the phase s etting accuracy of the digital phase shifters and on the accuracy of the op eration unit that calculates the phase to be set in the phase shifters. The accuracies (expressed as numbers of bits) required by the phase shifters a nd the operation unit so that they can produce the minimum change of the be am direction are investigated, and a simple formula is obtained. In order t o verify the results of these investigations, experimental results and nume rical calculations are presented. (C) 2000 Scripta Technica, Electron Comm Jpn Pt 1, 84(1): 68-76, 2001.