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Resonant Analysis and Application Research of Small Antena Coupling with Metal Cavity and Head Model

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Tutor: NiuZhongQi
School: Xi'an University of Electronic Science and Technology
Course: Electromagnetic Field and Microwave Technology
Keywords: Small Antenna,Resonant Cavity,Transmition Cavity,Resonant Frequency,Quality Fact
CLC: TN820
Type: PhD thesis
Year:  2010
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This dissertation mainly focuses on small antenne and resonant cavities. Small antennas are widely used in civilian and military fields. The antennae can be miniaturized using high dielectric constant materials, shorted load, prolonged current path or new-style left hand materials. Resonant cavities are used in the design of filter, frequency measurement, medium measure and so on. Beginning with the theory analysis, some electromagnetic phenomena were explained. Secondly, simulations and experiments are executed to validate the efficiency of the theory. Finally, the test system was established. Theories and methods used in this dissertation includes transmission line cavity theory, complex phase angle theory, modal expansion theory and so on. In addition, numerical methods such as Finite Element Method(FEM), Finite Difference Time Domain Method(FDTD) and HFSS software are employed.The author’s major contributions are as follows:(1) Performances variation of transmission line cavity was studied when which was loaded with antenna, including resonant frequency and quality factor. According to the parameters variation of cavity loaded pre and post, equivalent circuit parameters of loaded antenna can be obtained. Cavity performances were analyzed theoretically under the transmission and cut off mode based on equivalent circuit parameters of loaded antenna. It is proved that there were two resonant modes. The resonant frequency of cavity was changed under the transmission mode. New resonant frequency was generated under cut-off mode because the antenna was stimulated by cavity, which was called generalized resonance in this thesis.The antenna and cavity models were established in HFSS software, and the theory mentioned above was validated by simulation. A set of test system was designed and constructed, and the correctness of the theory was validated further. Because the loaded antenna was influenced by its loaded circumstance such as patch height, location of coupling probe, there would be an offset between the resonance frequencies in the cavity and frees space. A relativity validation experiment was designed and carried out, which proved that resonant frequency of cavity was linear correlation with the frequency of antenna.(2) Characteristics of antenna would be changed when it was put into metal cavity. Theory analysis shows that the input impedance varies because the coupling between antenna and cavity. If the cavity is big enough, the input impedance of antenna changes violently at the resonant point, but it changes little at the nonresonant point. If the cavity is small, it can influence the surface distribution current of antenna, which can lead to further effect on the input impedance.The modal expansion method was applied to get the equivalent circuit of input impedance of antenna which was put into the cavity, and the equivalent circuit was analyzed. This method was used to measure the antenna efficiency, but due to the coupling between antenna and cavity, the application range of wheeler method was restricted, in other words, it was only suitable for narrowband or single frequency point measurement. A set of cavity which had a variable resonance frequency was established based on the analysis mentioned above, which was called sliding shorted cavity. The size of the cavity can be changed by the movement of one end of the cavity, and resonant frequency varied consequently. The sliding shorted cavity combined with movement controller and vector network analyzer is used to construct measure system for antenna efficiency. A series of antennae were measured with the system. The comparison of the results between this method and standard method shows the method presented in this thesis is fast and valid.(3) The coupling between dual-band PIFA antenna and human head model was studied. The problem of mobile phone radiation has been widely concerned, and corresponding testing standards and testing methods were constituted. In this thesis numerical method (FDTD) was carried out to analyze this problem.Dual-band PIFA antenna model was established firstly. The characteristics in free space, such as standing wave characteristics, pattern and efficiency, were studied. Then it was placed at one side of human head phantom (Imm×1mm×1mm)and analyzed again. The characteristics measured in the free space and at one side of human phantom model were compared, and the effect of human head on antenna characteristics were obtained. Another study subject was the effect of antenna on human head, which was the problem of electromagnetic wave absorbed by human head. Distribution SAR in human head of dual-band PIFA antenna, including average SAR of lOg tissue, average SAR of 1g tissue, whole average and maximum was studied at the frequency of 900MHz and 1800MHz. Finally, the calculated SAR was compared with corresponding standard, and some methods and advices were given to reduce SAR.
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