Development and modelling of new wideband microstrip patch antennas with capacitive feed probes

Abstract

The principal contributions of this study include the development of a new capacitive feeding mechanism for wideband probe-fed microstrip patch antennas as well as the implementation of a spectral-domain moment-method formulation for the efficient analysis of large, but finite arrays of these elements. Such antenna configurations are very useful in the wireless communications industry, but extremely difficult to analyse with commercially available software. Probe-fed microstrip patch antennas have always been a popular candidate for a variety of antenna systems. Due to their many salient features, they are well suited for modern wireless communication systems. However, these systems often require antennas with wideband properties, while an inherent limitation of probe-fed microstrip patch antennas is its narrow impedance bandwidth. This can be overcome by manufacturing the antenna on a thick low-loss substrate, but at the same time it also complicates things by rendering the input impedance of the antenna very inductive. In this thesis, a new capacitive feeding mechanism is introduced that can be used for probe-fed microstrip patch antennas on thick substrates. It consists of a small probe-fed capacitor patch that is situated next to the resonant patch. The benefits of this configuration include the fact that only one substrate layer is required to support the antenna. It is also very easy to design and optimise. The use of full-wave methods for an accurate analysis of microstrip antennas, has basically become standard practice. These methods can become very demanding in terms of computational resources, especially when large antenna arrays have to be analysed. As such, this thesis includes a spectral-domain moment-method formulation, which was developed for the analysis of probe-fed microstrip patch antennas or antenna arrays that comprise of the new capacitive feeding mechanism. Here, entire-domain and subdomain basis functions are combined in a unique way so as to minimise the computational requirements, most notably computer memory. It is shown that, for general antenna array configurations, memory savings of more than 2500 times can be achieved when compared with typical commercial software packages where only subdomain basis functions are used. Some of the numerical complexities that are dealt with, include various methods to evaluate the spectral integrals as well as special algorithms to eliminate the recalculation of duplicate interactions. The thesis also contains a quantitative comparison of various attachment modes that are often used in the moment-method modelling of probe-to-patch transitions. Various numerical and experimental results are included in order to verify the spectral-domain moment-method formulation, to characterise the new feeding mechanism and to illustrate its use for various applications. These results show that, in terms of accuracy, the spectral-domain moment-method formulation compares well with commercial codes, while by comparison, it demands very little computer memory. The characterisation results show that the input impedance of the antenna can be fully controlled by only adjusting the size of the capacitor patch as well as the width of the gap between the capacitor patch and the resonant patch. In terms of applications, it is shown how the new antenna element can effectively be employed in linear arrays with vertical polarisation, horizontal polarisation or dual slant-polarisation. These represent some widely-used configurations for modern base-station antennas.