Design And Optimization Of Electrically Small Helical Antennas Based On Metamaterial Structures And Properties

Antennas play an important role in modern wireless communication systems. With the rapid development of wireless systems, there is an increasing demand for antenna miniaturization. As an important type of antennas with compact size, electrically small antennas with good impedance matching and radiation properties are of significance. Based on a thorough investigation of the theory on both electrically small antennas and metamaterials, in this dissertation, the design and improvement for normal mode helical antennas(NMHA) have been studied. The contents of this dissertation are as follows.In Chapter 1, the research background is introduced. The research history of the electrically small antenna theory, and some work on antenna miniaturization based on metamaterials in recent years, have been reviewed.In Chapter 2, two parts of important theories for the dissertation, the classic model and some improved models for normal mode helical antennas and three methods for solving the dispersion of composite right/left handed(CRLH) transmission lines, have been reviewed.In Chapter 3, the radiation properties of the helical antennas with non-uniform surface current distribution have been analyzed and the corresponding analytical results have been given. The analytical results have been compared with full-wave simulation of several helical antennas with typical size. Based on near filed coupling, a NMHA, with non-uniform current distribution and a half wavelength as its total length, have been designed. The working mechanism and the method for impedance matching have been clarified with its equivalent circuit model. Satisfying radiation efficiency and circular polarization have been obtained, as the size of the NMHA is characterized by ka of 0.2.In Chapter 4, in order to design NMHA with uniform surface current distribution, the parallel plate transmission line with peoriodically loaded capacitors, have been analyzed, based on the theory of CRLH transmission lines. The dispersion relation and particularly the zero-phase-shift frequency have been obtained. Based on such a structure, a bifilar circular cylindrical NMHA and square cylindrical NMHA operating at two independent frequencies, with 0.28 and 0.31 as their characteristic size, respectively, have been designed. The uniform current distribution along the antenna surface, the good impedance matching and circular polarization have been achieved. Meanwhile, the Q value of these NMHAs is to some extent approaching the Q limit for circularly polarized antennas with the same size.In Chapter 5, a preliminary study on the stored energy of the electrically small loop, as an important part of the classic electric-magnetic dipole model for the NMHA, has been conducted. The stored energy, both inside and outside of the circumscribing sphere, has been calculated, with a numerical integration for the magnetic potential vector in the classic loop antenna theory. The accuracy of our calculation has been compared and validated both by some previously reported and full-wave simulated results. In addition, cases when the ideal double-negative(DNG) metamaterials is filled within the loop has also been studied, and it is found that stored energy inside the circumscribing sphere and the resultant Q value of the antenna can be reduced significantly, when the DNG metamaterial has constitutive parameters of some specific value.In Chapter 6, the research work in the dissertation is summarized and some future work has been expected as well.