X-BANTTA MINKOWSKI YANSITICI DİZİ ANTEN ANALİZ VE TASARIMI
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Reflectarray antennas known with their ability of getting together successfully, capability of beam steering of conventional phased arrays and good efficiency and ease of design features of reflector antennas, are one of the most popular topics in recent years. Fractal geometries, if iterated infinite times, provide infinite circumference at a finite reach. This special property opens an area of development for reflectarray antennas in terms of miniaturization. In the literature review, it is observed that for reflectarrays using Minkowski fraktals which is the most widely used fractal in antenna engineering, could not reach one cycle phase range. In this work, miniaturized reflectarray antennas are produced by using Minkowski fractals as main reflector. In order to achieve a phase range more than a full cycle, a single layer with delay stubs and a multilayer structure with aperture coupled stubs are employed. In the thesis, Green’s Function implementation for planar surfaces and infinite array approach that is used in analyzing reflectarray antenna unit cells are elaborated as well as reflectarray antenna theory formulations. During the work, for single layer reflectarray, it is observed that single stub option causes high cross polarization levels, therefore usage of two stubs with orthogonal symmetry is proposed to overcome the problem. In order to show the validity of the proposal, unit cell samples produced in Hacettepe University Electrical and Electronics Engineering Printed Circuit Laboratory are measured by waveguide measurement technique. Simulation and measurement results are in concordance and the reliability of proposed setup is proved. In unit cell design, an optimization procedure is prescribed and implemented for both single and multi layer antenna structures. Proposed procedure works like, among all parameters that are subject to optimization, one is chosen at each time and simulated to find total phase range, amplitude and an error value which decreases as the phase curve approaches to a target line. The optimum value is chosen and designated in the software for the following simulations. First iteration is completed after all parameters are defined. In case not enough linearity, phase range and/or amplitude could be reached, further iterations are conducted. Unit cells are simulated and designed for predefined dielectric materials by using Ansys® HFSS™ software. One full cycle for single layer and more than two cycles for multi layer structure is obtained. Then stub lengths are calculated for 221 elements of the antenna depending on predefined incident wave direction and feed antenna position. Full size antenna is fabricated using acid treatment method. Fabricated single and multi layer antennas are measured in unechoic chamber of Middle East Technical University Electrical and Electronics Engineering Antenna Research Laboratory. Significant similarity is observed in main beam shape, half power beamwidth, side lobe levels and notch positions between measurement and simulation results. It is observed that side lobe levels and half power beam widths of single and multi layer antennas are close to each other. However, multi layer antenna has 1.1 dB better gain and 5 % more aperture efficiency. 10.9% miniaturization is realized by single layer antenna. On the other hand, although the thickness is increased five times in comparison with single layer structure, multilayer antenna achieves 21.9% miniaturization.
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