| Summary: | Ultra-wideband (UWB) technology is a promising solution to provide high data-rate transmission of the future wireless communication systems. The rapid development of UWB wireless communication systems has brought both challenges and: opportunities to novel UWB antenna designs. This thesis is concentrated on the analysis, design and measurement of compact antennas for UWB devices, which can be divided into three areas. / The first area investigated is small planar broadband antennas for wearable and positioning applications where the antennas with unidirectional radiation patterns may be preferred, since the energy will be focused on the desired direction. Furthermore, the interference from the environment behind the antenna can also be greatly minimised. Two low-profile broad band antennas with unidirectional radiation patterns are presented in this work. Their performance in terms of impedance bandwidth and radiation patterns is studied. Results show that they have a very broadband (>50% fractal bandwidth) and constant broadside unidirectional radiation patterns. Hence, they are good candidates and suitable for the in-body and radar applications The second research area is about the planar UWB monopole antennas. The radiation behaviour of the planar UWB monopole antenna is first investigated. It is found that an ultra-wide bandwidth of this type of antenna is achieved by both well-matched travelling wave modes and smooth transients between different higher order modes. However, there exists a common problem for the current planar UWB monopole antennas that their performance is heavily affected by the size of the ground plane. It is found that dimensional changes will influence the characteristic modes excited on the ground plane which directly affects the impedance and radiation of the antenna. A technique of cutting slots on the ground plane is then proposed to resolve the ground plane effect problem. The presence of the slots will introduce slot modes. Optimal slots can produce a strong coupling between the slot mode and the ground plane modes. This coupling makes the characteristic modes of planar UWB monopole insensitive to the size change of the ground plane, hence it effectively minimises the ground plane dependence of the antenna. Two different UWB monopole antennas are investigated using this technique. Both the simulated and measured results demonstrate that this method can indeed minimise the ground plane effects on the impedance matching and have very little effect on the other antenna parameters, such as radiation pattern, gain and I time domain performance. Additionally, a parametric study on the major geometric parameters related to the slots is performed to achieve an optimised design. The third area of my study is the measurement of UWB antenna radiation efficiency. Antenna radiation efficiency is one of the most important parameters when evaluating its performance. In the past, many different antenna efficiency measurement methods were proposed. However, how to measure the UWB antenna efficiency effectively is still a challenging issue. A novel source-stirred chamber/cap (SSC) method is proposed and developed to overcome the limitations of the existing methods. Different types of UWB antennas are employed as examples to demonstrate the usefulness of this new method. Promising results are obtained which prove the SSC method is very effective in measuring the UWB antenna efficiency. It is evident that this method has the potential to become a general antenna efficiency measurement approach.
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