| Ultra-wideband (UWB) antenna techniques applied in the emerging UWB wireless communication systems are deeply studied in this dissertation. UWB techniques have typical characteristics including large bandwidth occupying several GHz and extremely short pulse transmitted without carrier. Considering the specialties of signals and practical application environment, UWB systems require the antennas with more rigorous electrical and physical performances than those of traditional narrow-band or wideband antenna. Hence, these conventional antennas will not be suitable in such systems anymore and new types of UWB antennas are desirable.In this dissertation, four aspects, including the basic design theories of UWB antennas, novel UWB antenna structures, the design of fractal UWB antenna, and time-domain UWB antenna arrays are the research emphases. The major achievements are listed as follows:Firstly, the requirements of UWB antennas are described. Based on the theoretical analysis of impulse and continuous signals in radiating and receiving procedures, the dominant aspects degrading antenna performances are revealed. Necessary parameters to evaluate the distortions of signal waveforms are also defined in time domain. Consequently, extremely large bandwidth, excellent waveform fidelity of signals, and compact size, are three fundamental requirements for an UWB antenna. Some primary conclusions of UWB antenna design are discussed briefly. These results provide the necessary clues in practical antenna design.Secondly, the bandwidth and feeding problems of existing planar elliptical antennas (PEDAs) are solved. With trimmed elliptical radiation patches and microstrip-coupled feeding structure, the shortcomings of the PEDAs can be overcome effectively. Based on these new ideas, a novel microstrip-fed PEDA (MF-PEDA) is proposed. MF-PEDA exhibits excellent broadband behaviors covering the frequency range from 3.1 to 10.6GHz, good fidelities of signal waveforms in space, and radiation patterns similar to those of a conventional dipole antenna. As a further improvement, a microstrip-fed semi-elliptical dipole antenna (MF-SEDA) is proposed. It has the dimension of only about one third of the wavelength instead of generally half wavelength for the lower frequency. In addition, with simple and proper modification of the radiation patches, a frequency-notched antenna based on the MF-SEDA can be constructed and provide good band-notched performance from 5.0 to 6.0GHz. These new types of antennas are very suitable for various UWB systems.Thirdly, several design methods of fractal UWB antennas are discussed. Fractal antennas have the characteristics of compact size and multi-band frequency response derived from the self-similar properties. A planar Koch fractal slot antenna with a microstrip- coupled feeding structure is designed. The Koch fractal slot results in a remarkably reduced size and good electric performances. Both simulated and measured results show that the simple antenna has broad bandwidth from 3.0 to 6.0GHz, approximately omni-directional patterns, and a dimension of about one fourth of the wavelength at the lower frequency edge. This compact antenna is very suitable for the integrated UWB systems.At last, the characteristics of novel two-dimensional Y-shaped and three-dimensional cone-shaped time-domain UWB antenna array are analyzed. Radiation performance, including the fidelity of waveform and directional energy patterns of radiated signals can be obtained by the change of array parameters. Results indicate these arrays have low sidelobes, the energy beam of radiated signals can be steered to arbitrary direction in space through the time delay configurations of each array element, and the optimal fidelity of waveforms can also be achieved in the maximum radiation direction at same time. Compared with the linear or planar arrays, the cone-shaped array may avoid the grating lobes and provide the unique maximum radiation direction in space. It is a good choice for the antennas in UWB wireless access point devices or base stations.
|
PDF Full Text Request