Design Guidelines, Scan Behavior and Characteristic Mode Analysis for a Class of Ultra-Wideband Microstrip Patch Antennas

Abstract

Ultra-wideband (UWB), low-profile microstrip patch antennas and phased arrays have their niche in many wireless communication and medical applications. In recent years, the U-slot patch antenna established itself as a versatile antenna that can be fine-tuned for ultra-wideband operations. The L-shaped probe feeding method has additionally led to improved impedance bandwidth for the U-slot patch antenna. The L-probe’s simple structure together with its low production cost makes it an attractive feeding method for the U-slot microstrip patch antenna. In phased arrays, scan blindness due to surface wave excitations can reduce the scan bandwidth range. By reducing the mutual coupling between array elements, the scan blindness effects will be reduced. Also, by reducing the sidelobe levels and minimizing the effect of grating lobes in phased arrays, the array’s scan performance and power efficiency can be improved. In this dissertation, (1) a parametric study is performed on εr = 2.2 and 4.5 substrates for the design of ideal L-probe feed dimensions with optimum impedance bandwidth. Results show that first-pass optimum impedance bandwidth of over 50% is achieved using the ideal L-probe feed dimensions. (2) The mutual coupling between a 2-element UWB microstrip array using different patch orientations and U-slot topologies is examined for εr = 2.2 and 4.5 substrates to reduce the effect of scan blindness. Results, for εr = 2.2 substrate, indicate that a diamond patch orientation with opposite U-slot topology presents the least coupling between the array elements. For εr = 4.5 substrate, the E-plane patch orientation with parallel U-slot topology has the least coupling. (3) The scan behavior of 5x5 planar phased arrays using different patch orientations and U-slot topologies is examined for εr = 2.2 substrate. Results indicate that blind spots are less prevalent in the diamond patch orientation and more prevalent in the E-plane patch orientation which has the most coupling between the array elements. (4) The array patterns of a 17-element L-probe-fed U-slot microstrip linear phased array are examined at different combinations of uniform and nonuniform excitation and inter-element spacing. Results indicate that using nonuniform excitation and inter-element spacing can reduce the sidelobe levels by over -10dB as the array is scanned 60° away from broadside. (5) Lastly, the Theory of Characteristic Modes (TCM) is used to characterize the resonant behavior of different microstrip patch shapes, substrates and excitation feeds to realize a microstrip patch antenna design with optimum broadband behavior. Results indicate that a single-layer U-slot rectangular patch on εr = 4.4 substrate presents a highly radiating structure. Further modal analysis of this single-layer structure with a single T-probe feed shows that VSWR ≤ 2 impedance bandwidth in the order of 96% can be achieved. Experimental results show VSWR ≤ 2 impedance bandwidth in the order of 71%.