الفهرس 
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Abstract
The problem of determining the current distriution along each of the elements of a circular array of dipoles and turnstile antennas due to a given excitation is treated as a boundary value problem. The current distribution is obtained by applying the method of moments. The obtained current distributions on the dipole elements are then employed to study the radiation pattern, gain and polarization characteristics of the dipole array.
One of the most phase dipole arrays is the turnstile antenna which is a circular array of dipoles whose elements are excited by voltages of the same amplitude but with a phase shift that advances from one elements are excited by voltages of the same amplitude but with a phase shift that advances from one element to the next. The dipole elements are arranged in circular arrays to give different forms of turnstile antennas which are required for satellite applications. The turnstile antennas investigated in the present thesis are the radial-dipole, tangential-dipole, crossed-dipole, and canted turnstile.
A crossed-dipole turnstile is designed to operate as a low-orbit satellite antenna. The radiation pattern of such an antenna can be easily shaped by properly setting the height at which the turnstile is placed above the satellite body. The radiation patterns with pure circular polarization required for telecommand and earth coverage applications for low-orbit satellites working in the UHF band are obtained.
The upper surface of the satellite body is represented as a square conducting plate. A wire gird model is used to represent the conducting plate. The validity of the wire gird model is investigated and the accuracy of the obtained results regarding the current distribution on the square plate is realized.
The frequancy response of the input inpedance and the VSWR of the crossed dipole turnstile antenna when it is left in free space as well as that when it is placed above a square conducting plate are presented and the bandwidth of the antenna is investigated. The frequancy band over which the turnstile antenna has a matched impedance is investigated.
The gain obtained from some forms of circular arrays of dipoles is maximized in a specific direction by optimizing the excitation voltages of the array elements so as to satisfy the gain maximization requirement. The dependence of the maximum gain on some geometric parameters of the array, such as the length of each dipole, the radius of the array, and the number of array elements is investigated.
A sidelobe reduction method is used to determine the array port excitations that will produce a far field radiated power density of a predetermined level in specified directions. For a unique radiated power density of a predetermined level in specified directions. For a unique solution, the number of specified directions in which the radiation level is to be controlled and the number of independent excitations are made equal.