Scattering Of Fractal Structure And Design Of Some Novel Fractal Antennas

With the rapid development of wireless technology, the communication system has stricter requirements for antenna design, such as miniaturization, multi-band performance, and circular polarization. Due to the continuous development of electromagnetic theory, there are many different ways to achieve miniaturization, multi-frequency, CP and broadband. Typically, an antenna with two or more features mentioned above is needed, such as: miniaturized CP antenna, miniaturized wideband antenna and compact wideband CP antenna, and the existing technology fails to achieve performance above. Therefore, it becomes necessary to use new theories and methods to explore modern antennas, in order to resolve the problems encountered with the designing of conventional antenna. Fractal antennas have space-filling and self-similar properties, and have natural advantages over conventional antennas. For instance, space-filling enables us to design small antennas effectively, and self-similarity makes it possible to achieve multi-band.This dissertation applies fractal theory to meet the new demands of communication systems and is supported by the National Science Foundation of China. It is entitled “Relationship between fractal antenna and fractal dimension and its application to the design of small antenna”, and it focuses on the relationship between fractal dimension and electromagnetic scattering characteristics, miniaturized CP antenna, dual-band RFID fractal antenna, fractal wideband antenna and miniaturized wideband CP antenna. Specially, the main contributions of this dissertation are summarized into the following five sections:1、The first section is discusses the electromagnetic scattering characteristics of fractal sphere. Multilayed fractal sphere is set on basis of Cantor set, and the RCS values in different directions are calculated. The results show that there are similar scattering characteristics for self-similar structure of fractal multilayered sphere. The bigger the fractal dimension, the larger the RCS is. In addition, it is shown that the backscattering is relatively sensitive to fractal dimensions.2、Second, two novel miniaturized CP Koch fractal antennas are proposed and designed. The traditional cross-dipole is replaced by Koch structure for size-reduction. It is found that the impedance of the antenna can be changed by adjusting the Koch fractal dimension, sothat the amplitude of current for two cross-dipole are equal and the phase differ 900, which leads to CP. Simulated and measured results show that this new type of miniaturized CP antenna has good radiation characteristics, 3d B axial ratio bandwidth of 5.98%, and offers 23.4-33.5% size-reduction from that of conventional antenna. These antennas were used for Chinese Beidou I vehicle satellite navigation system successfully.3、In the third section, a novel dual-band fractal RFID antenna is proposed. The equivalent circuit model is used to adjust the length and thickness of the branches, and two tree-fractalarray metamaterial structures are designed. To study the properties of structure, S-parameters are used to derive permeability and permittivity of the structure. The results show that the real part of permeability is negative. The designed fractal structures were placed between the patch and ground, and dual-band was achieved by using MNG property. The tested results show that the antenna resonates at 842 MHz and 922 MHz, with bandwidths of 4.4% and 3.1%, respectively; hence they fully cover two Chinese RFID/UHF frequency bands.4、In section 4, a wideband printed slot antenna based on Koch fractal. Self-similarity and space filling is used to enhance the bandwidth of slot antenna. The Koch fractal was used for the edge of the antenna and simulated results show that by choosing appropriate IO, IF, and IA, the bandwidth can be enhanced. In addition, eight slot antennas were fabricated by using the optimized parameters, their performances were compared and the optimal broadband slot antenna was designed. The measured results show that the antenna can achieve 1.75 octave(1.45-4.86GHz), with a stable pattern and maximum gain of 5.38 d Bi, which is very suitable for modern wideband wireless systems.5. Finally, a novel miniaturized broadband(2-6GHz) fractal CP antenna is proposed. Different from the traditional phase shifter CP antenna, the CP is achieved by optimizing the fractal dimension of the dipole mentioned in Section 2. Meanwhile, combining with broadband characteristics of LPDA, the miniaturized wideband CP can be obtained without phase shifter and complex matching circuits. The test results show that the antenna has good CP, the antenna gain in the low and high frequency bands are 4d Bi and 6d Bi, respectively. This antenna meets the requirements of missile-borne antennas of the Space Center.