Design Of High Isolation MIMO Antennas Based On The Characteristic Mode Method

With the popularity of the fourth-generation mobile communication,the capacity of wireless channel,communication quality and transmission distance have been greatly improved by using the MMO technology without occupying additional spectrum resources.These advantages make MIMO technology have attracted much attention in recent years.However,since the space of the terminal equipment is small,the high coupling between the antennas seriously affects the performance of the antennas.Therefore,it is necessary to design a miniaturized and highly isolated MIMO antenna.In recent years,the advantages of using the characteristic mode theory to analyze and design the antenna have been more and more obvious.It can study the radiation mechanism of antennas from the physical essence without dependency on the feed.Also,it can effectively guide the antenna design by analyzing the characteristic modes of the antenna.What is particularly important is that the orthogonality between the characteristic modes provides an effective solution for the design of a highly isolated MIMO antenna.The main work of this paper is summarized as follows:1.It corrects the error in the tracking of characteristic mode when it is performed in the broadband.First,the commercial electromagnetic simulation software is used to model the antenna and obtain the impedance matrix.Then,by using the MATLAB programming language,the eigenvalues and eigenvectors are calculated from the obtained impedance matrix at the sampling frequencies,and then the relation matrix of the characteristic modes between the adjacent frequencies is calculated by the characteristic mode tracking algorithm.From the element values in the relation matrix,the corresponding relationship between characteristic modes at the adjacent frequencies is determined.The characteristic mode tracking results are obtained at all the sampling frequencies ultimately.2.It explores the excitation methods of the characteristic mode.By capacitive and inductive loading,the corresponding characteristic modes are excited respectively.According to the characteristic mode analysis of the mobile phone board,a four-port MIMO antenna is designed successfully.The main antenna is designed by exciting the wide bandwidth mode 1,and the auxiliary antenna is designed by exciting the mode 4 and the mode 2 respectively.By the measure that different antennas work in different orthogonal modes,the isolations between antenna ports are all above 10 d B.3.By modifying the structure of the mobile phone board,a two-port MIMO antenna working at less than 1GHz resonant frequency is designed.The resonant frequencies of mode 2 and mode 4 are shifted below 1 GHz by loading the frame.The main antenna works in the wide band mode 1,and the auxiliary antenna works in mode 2 and mode 4 at the same time.The isolation between the main antenna and the auxiliary antenna at the low frequency is more than 9d B since they work in different orthogonal modes.