Applications Of Characteristic Mode Theory In Antennas Of Wireless Communication System

The theory of characteristic mode?CM?only relate to structure,physical dimensions,and electromagnetic material properties of the target object,regardless of any particular excitations.As compared to traditional antenna designs,the design based on the CM theory could not only provide the clear physical meaning to the radiation principle and scattering mechanism of the target objects,but also provides a greater degree of freedom in terms of design,which is no longer dependent on personal experience.In this thesis,the design method based on the CM theory is used to optimize and control the radiation and scattering properties of antennas.The main contents of this dissertation are as follows:1.Firstly,the development of CM theory is briefly introduced,which lays the foundation for the following work.The generalized eigenvalue equation is derived by introducing an impedance operator that relates the current to the electric field generated by the current.Then,the orthogonality of characteristic currents and fields are proved.Based on the orthogonality,the surface current of the perfectly electrically conducting bodies can be expanded by characteristic currents.The definitions and physical meanings of eigenvalues,modal significances and characteristic angles are also described.Finally,the complete discretization process of the method of moment?MoM?is given.2.A novel stacked antenna configuration is proposed for the designs of shared-aperture base station antennas.In addition,the antenna design is based on the CM theory.An antenna array consists of a lower-band antenna working in 690-960 MHz frequency band and four upper-band antenna elements operating in the 3.5-4.9 GHz band is designed to demonstrate the concept.In the proposed configuration,the upper-band antenna elements are placed over the aperture of the lower-band antenna.Four metallic sheets are introduced to provide capacitive loading for the lower-band antenna and simultaneously offer a ground plane for the upper-band antennas.Therefore,low-profile characteristic and high isolations between the dual-band antennas are guaranteed.The overall height of the antenna array is only 0.17?_L0(?_L00 is the free-space wavelength at the center frequency of the lower-band),which is much lower than those of conventional designs.CM analysis indicates that two modes in the lower band are combined and excited to expand the bandwidth of the proposed lower-band antenna.A prototype of array antenna is fabricated to validate the proposed design.The measured results for the two bands are in good agreement with the simulated results.Owing to its simple structure and compact size,the resultant antenna array is easy for fabrication and has great potential in 5G MIMO communication systems.3.A methodology for achieving in-band radar cross section?RCS?reduction of antennas is proposed.The methodology for the analysis and control of scattering from antennas can be divided in three steps:?a?Carry out the CM analysis for the antennas to get modal significance,modal currents,and modal weighting coefficient of each mode.?b?Distinguish between radiation modes and dominant scattering modes based on the characteristic mode intrinsic parameters.?c?Reduce modal weighting coefficient of dominant scattering modes based on the modal current distributions of the scattering modes.The proposed method is used to reduce the RCS of the microstrip antenna and the slot antenna to verify its feasibility.Following the specific mode characteristics,two techniques are proposed to reduce the MWC for each scattering mode.The first technique is to shift the resonant frequency of a particular scattering mode out of the operation frequency band.The second is to disrupt the current path and thus reducing the bandwidth of the scattering mode.A prototype of the slot antenna is fabricated to validate the proposed methodology.Measured results agree well with the simulated ones.