Multi-objective optimization of antennas for ultra-wideband applications
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There are a growing number of ultra-wideband applications, which involve the radiation or reception of electromagnetic signals over frequency bandwidths ranging from 1.3:1 to over 10:1. In the design of antennas for ultra-wideband systems, many design objectives must be considered, including impedance matching, radiation efficiency, radiation pattern stability, size, and possibly impulse response. Given the very wide bandwidths considered, it can be challenging to meet all objectives simultaneously, and optimization techniques are useful to achieve a reasonable compromise between objectives. In this dissertation, multi-objective Genetic Algorithm (GA) optimization is used to design ultra-wideband antennas for use in wireless communications and low frequency radio astronomy. GA optimization is first applied to the design of ultra-wide bandwidth planar monopole antennas, which exhibit a narrow-band frequency notch in order to mitigate interference with co-located radio systems. The GA optimizer uses a weighted sum cost function related to impedance matching and radiation patterns at frequencies within both the wide operating band and the narrow notch band to improve antenna performance. A two-dimensional matrix chromosome is used in the GA to represent a wide-range on planar element shapes. It is shown that the GA generates antenna designs which exhibit wideband performance equal to traditional band-notched designs, but have improved azimuth plane radiation pattern symmetry, which widens the effective notch bandwidth. Pareto GA optimization is then applied to the design of planar dipole antenna elements operating over a ground plane for use in a low frequency radio telescope array. The objectives considered include Galactic background or "sky noise reception level, and radiation patterns over the operating band of 20 to 80 MHz. It is demonstrated that the Pareto GA approach generates a set of designs, which exhibit a wide range of trade-offs between the two design objectives, and satisfy all applied geometrical constraints. Multiple GA executions are performed to determine how antenna performance trade-offs are affected by different geometrical constraint values, feed impedance values, radiating element shapes and orientations, and ground conditions. In a follow-up to the previous study, the effects of mutual coupling in a low frequency radio telescope array are considered. It is first shown that a simple receive-based definition of coupling between two antennas can be used to design antenna elements which exhibit reduced mutual coupling effects when operated in a large phased array. This result is utilized in order to perform Pareto GA optimization of wire frame bow-tie dipole elements in terms of mutual coupling, as well as sky noise response and radiation patterns over the 20 to 80 MHz band. The GA generates a set of designs that span a wide range of objective values. The results are analyzed to understand the trade-offs that may be made between the three objectives.