| The rapid development of communication system requires more and more on the antennas'design. The miniaturization of the communication equipments calls for miniaturized antennas, while the increase of the communication frequency bands puts forward the development of multi-band and ultra-wideband (UWB) antennas. UWB antennas cover wide frequency range which avoids the communication systems to equip several antennas and they instead decrease the size of the facility and improve the electromagnetic compatibility. Meanwhile, it is demanded sometimes that the antenna can receive signals at appointed frequency points to reduce the interference. Under such circumstances, multi-band antennas can meet this demand. Thus, the research work of the antennas nowadays focuses on the miniaturization, multi-band and ultra-wideband characteristics.There are many methods proposed to realize the miniaturization and multi-band characteristic of the antennas. However, these methods are so specialized that only some aspects of the demand can be met while the overall performance of the antenna is decreased to satisfy the communication system. To solve this dilemma, this dissertation proposes miniaturized, multi-band and wideband antennas based on fractal structures theory and discusses the problems during the research of the antennas'design as follows:In the first place, this dissertation discusses the basic theory of fractal antennas. As antennas are passive elements, the electrical characteristics of them are decided by the structure mainly. Generally, fractal structure has two special properties: space-filling property and self similarity property which may help the antenna have good performance. The fractal antennas such as symmetrical dipole, circle antennas and microstrip patch antennas, are analyzed and simulated to discuss the relationship between space-filling property of fractal structure and the miniaturization characteristic, the formula to describe the miniaturization of antennas is proposed. Through the simulation of fine structure of Koch-type fractal antennas, the propagation characteristic of the surface current on the fine structure is discussed which can explain that high order fractal structure not always leads to miniaturization antenna. By the simulation of the return loss and the surface current for the Sierpinski-type fractal antennas, it is discussed the relation of self similarity property and the multi-frequency band characteristic, and it has been concluded that the frequency ratio is related to the similarity ratio of the fractal structure. Further, the periodical logarithm antennas with different ratios are simulated and the relationship between self similarity property and ultra-wideband characteristic is discussed. The above research work provides theory background and technological methodology for the design of miniaturized fractal antennas.Secondly, miniaturized fractal antennas are proposed in this dissertation. The application of the fractal structure can be practiced solely on the antennas'design or with other methods. A miniaturized Yagi antenna is designed with the combination of fractal structure and ground plane, and size of the antenna is reduced to 61% compared with traditional Yagi antenna. A miniaturized wideband fractal antenna is proposed with the combination of fractal theory and methods of widening frequency-band. Based on such methods, a microstrip fractal antenna with double patches is designed. The antenna has 11% of the relative impedance bandwidth and the size of it is reduced by 17%.The third aspect of the research lies in the fractal antennas with multi-band characteristic. The nesting fractal structure provides good multi-band characteristic of the antenna. Based on this idea, a method to design multi-band antennas is proposed. The metal sheets with various shape and size comprise multi-band antennas with nesting structure. It is proved that the number of the pass-bands is decided by the number of nesting units and the central frequency of each pass-band is related to the shape and size of the nesting metal sheets accordingly. Three forms of fractal antennas with three pass-bands are designed and the control of the frequency is flexible. The radiation properties at the three pass-bands are almost the same which meet the demand of multi-band characteristic.The miniaturized wideband fractal antennas are discussed in the fourth part. The periodical logarithm antenna is considered as a fractal array, the units and separation of the array are both fractal structures. Through the fractal evolution of the units of the array, a miniaturized periodical logarithm antenna with tree-fractal structure is proposed. The size is reduced by 38% at the polarized direction. Furthermore, based on the structure of the antenna and the condition of independent frequency, a composite triangle fractal antenna of the 51% normal size is designed which has a multiplication frequency bandwidth of 19:1. The measure experiments meet well with the simulation results.This dissertation proposes several principles and methods to design miniaturized, multi-band and ultra-wideband antennas based on fractal structures. The antennas are made and measured. The conclusion in this dissertation provides consultation and results for the practice of antennas'design.
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