Design Of Compact Ultra-Wideband Antennas And Arrays

Based on modal analysis, a novel method to evaluate the physical limits of antennas and the miniaturization of ultra-wideband (UWB) antennas are presented in this dissertation. Both antenna elements and arrays can be designed with high performance.The field distribution outside the surface of arbitrary shape that encloses an antenna can be represented by equivalent sources on that surface. The surface integral equations in terms of these equivalent sources are derived and discretized by the method of moments (MoM). The relations of the radiation power and the stored energy to Q value and the maximum gain of the antenna are established and expressed in matrix forms. The physical limits of these antenna parameters are obtained by solving an optimization problem with constrained condition. Such an optimization problem can be tackled as a linear algebraic general eigenvalue problem. By this method, the fundamental modes of the equivalent sources can be determined and the lowest Q value and maximum gain to Q ratio are obtained. The method can also take into account complex electromagnetic environments, such as a conducting plane of finite size, in computing the physical limits of the antenna. It is applied in the miniaturization of UWB antennas.Characteristic mode theory is utilized to calculate the lowest order characteristic modes and their frequency responses in the analysis and design of compact UWB antenna, such that the relationship between geometric structure and antenna performance is visualized. Since the eigenvalue of the resonant current mode approaches zero, the reactance needed for shifting the resonant frequency can be computed directly and constructed by distributed reactance loading, such as stubs and slits. The significances of the related modes are tuned and their frequency responses are optimized. As this method may deeply show the mechanism of a wideband or multimode antenna and obtain the values of loaded reactance directly, the dependence on time-consuming full wave simulation can be reduced and the design process accelerated.A wide slot UWB antenna with a very compact size is presented in this dissertation based on characteristic mode analysis. With added tuning stubs, the antenna can achieve a wide band performance with a low profile. To avoid interference to existing wireless systems, such as WLAN and WiMAX, dual notched bands are constructed by Hilbert fractal curve or inverted U-shaped slit and C-shaped slit. However, the higher order harmonic waves of these slits produce a spurious stop band falling in the operation band. This phenomenon is investigated by modal analysis in detail and the spurious notched band is canceled by adding a rectangular patch.In this dissertation, a compact tapered slot antenna with single microstrip feed line is designed to obtain uni-directional radiation patterns. Several shorting strips are added to the antenna to decrease backward radiation. A tapered microstrip line and a sector-shaped stub are employed to improve the impedance matching. With these efforts, the antenna performs directional radiation over the band. Based on the above single-fed antenna, a differential directional antenna and its BALUN are also designed. The current distribution of the differential antenna is symmetric and not significantly affected by the coaxial cables used at the input for connection propose. The antenna can be connected to a differential front-end circuit directly.A fast method based on the method of moment is proposed for the analysis of UWB antenna arrays, in which mutual coupling effects can be considered. When a large-scale array is to be analyzed, it is difficult to solve the problem by the conventional MoM because of the large order matrices involved. Characteristic modes are utilized to expand the current distribution on each individual antenna element. As only the resonant modes significantly contribute to radiations, the order of the matrix equation can be reduced and the computational efficiency is enhanced. By this method, the modal patterns and exciting voltages at input ports are computed. Several UWB antenna arrays with mutual coupling are analyzed. Far field radiation patterns are synthesized and optimized using the modal patterns. The scanning angle is also optimized.