Authors: Kalyan K. Karnati
Source: FERMAT, Volume 7, Communications 2, Jan Feb.,2015
Abstract: Reflectarray antennas uniquely combine the advantages of parabolic reflectors and phased array antennas. Comprised of planar structures similar to phased arrays and utilizing quasi-optical excitation similar to parabolic reflectors, reflectarray antennas provide beam steering without the need of complex and lossy feed networks. In the presented research, a novel theoretical approach to extract the reflection coefficient of reflectarray unit cells is developed. The approach is first applied to single-resonance unit cell elements under normal and waveguide incidences. Using this theory, effects of different physical parameters on reflection properties of unit cells are studied without the need of full-wave simulations. Detailed analysis is performed for Ka-band reflectarray unit cells and verified by full-wave simulations. In addition, an approach to extract the Q factors using full-wave simulations is also presented. Overall, novel theoretical approaches to analyze the reflection properties of the reflectarray elements using Q factors are developed. The presented theoretical models provide valuable physical insight utilizing coupling conditions and aid in efficient reflectarray design. In addition, for the first time a continuously tunable reflectarray operating at Ka-band is presented using BST technology. Due to monolithic integration, the technique can be extended to higher frequencies such as V-band and above. Later, the Q factor theory is extended to account for the varying incidence angles and polarizations utilizing Floquet modes. Emphasis is laid on elements located on planes where extremities in performance tend to occur. The antenna element properties are assessed in terms of maximum reflection loss and slope of the reflection phase. A thorough analysis is performed at Ka band and the results obtained are verified using full-wave simulations. Reflection coefficients over a 749-element reflectarray aperture for a broadside radiation pattern are presented for a couple of cases and the effects of coupling conditions in conjunction with incidence angles are demonstrated. Finally, tunable reflectarray elements capacitively loaded with Barium Strontium Titanate (BST) thin film are developed. The effects of substrate thickness, operating frequency and deposition pressure are shown utilizing coupling conditions and the performance is optimized. To ensure minimum affects from biasing, optimized biasing schemes are discussed. The proposed unit cells are fabricated and measured, demonstrating the re-configurability by varying the applied E-field. To demonstrate the concept, a 45 element array is also designed and fabricated.
Keywords: Reflectarray, Q factors, coupling conditions, waveguide analysis, Floquet modes, reconfigurable antennas, BST, micro-fabrications, beam-steerable antennas, monolithic integration.
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