الفهرس 
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Abstract
Antenna arrays play an important role in electronic warfare, e.g. jamming can be avoided by using low sidelobe antennas. The thesis investigates planar phased arrays of flat and cylindrical dipoles in free space backed, generally, by a ground plane. The study covers mainly the active impedances of the different elements of the array and their change with scanning in both the E- and H- planes. The arrays considered produce either pencil beam for tracking radars or fan beam for search radars (cosecant pattern). The current distributions considered include a uniform distribution, and tapered distributions (pedestal and Taylor ) to produce low sidelobe levels. The ”Element-By-Element” method (EBE) is used to study the array with current sources of zero internal impedance. It is found for broadside radiation that the elements active impedances change slightly when using uniform or tapered current distributions on the array for pencil beams. The impedance variation with scan angle is reduced by appropriate choice of the array distance to the ground plane. The computation of the mutual and self impedances for strip dipoles is based on computing double infinite integrals with oscillating functions, which is time consuming, particularly for large arrays. Their accuracy and speed of computation are investigated using two methods. The first method is the ”Finite Periodic Structure” (FPS) method, which is used for the analysis of large arrays, with a study made to avoid errors occurring in computing impedances of edge elements. Another technique is used to reduce the computation time which uses the conventional element-by-element method with neglecting the mutual coupling to far elements. The convergence of the integrals in the two methods is studied. Results are obtained showing the accuracy and the computation time, for which approximate formulas are obtained as a function of the array dimensions. The active impedances of dipole arrays are investigated with generators having internal impedances. The impedances of the elements and the current distribution on the array show the presence of an interference pattern between the forced scanned wave excited by the generators and another wave traveling on the antenna surface. For the case of short non-resonant elements with small separation, the traveling wave is known to be a surface wave. It was found that in addition to this surface wave, another surface wave with larger propagation constant exists. As the element spacing increases the propagation constants of the two surface waves approach each other, then merge into a single root, which becomes a complex leaky root. These surface waves interpret the interference pattern found for the currents on the array. The case of the surface waves traveling on the array with resistive load was studied. The interference pattern on arrays with resonant elements and half wavelength spacing was found to be due to another type of leaky waves. Excitation of the array with tapered excitations and fan beams was considered using generators with internal impedance. High and low active impedances at the array edges were found with monotonic variation of the active impedances along the array. The elements impedances on both edges and at corners of the array were investigated.