| Metamaterial is a kind of equivalent homogeneous artificial composite structures.It has novel physical properties and cannot be obtained directly in nature.Metamaterial greatly broadens design ideas of modern antenna and effectively improves the antenna performances.Especially for the antennas used in 5G or post 5G wireless communication systems,metamaterial has a promising application in the antennas,including array antennas with high isolation,wideband antennas and reconfigurable antennas.Therefore,based on the two-dimensional structure of metamaterial which is named metasurface,the dissertation mainly studies how to utilize the metasurfaces properly for improving the antenna performances.The major results and contents are shown as follows:1.The problem how to effectively increase the isolation of a closely spaced array antenna is discussed.First,based on studying surface wave suppression characteristic of EBG(Electromagnetic Band-Gap)structures,a UC-EBG(Uniplanar Compact Electromagnetic Band-Gap)structure is designed and applied to an array antenna where the adjacent antenna element spacing is 0.5?.It is found that compared with the reference antenna,the port isolation of the antenna with UC-EBG structures is improved by 13 d B.Moreover,to further reduce the mutual coupling,three cross slots are etched on the ground to disturb the surface current.Finally,the results demonstrate that contrasting the reference antenna,the port isolation of the proposed array antenna is increased by 16 d B,and the two decoupling structures have a little effect on the antenna radiation performance.Additionally,compared with some array antennas using decoupling superstrates or traditional EBG structures,the proposed antenna not only achieves high port isolation but also keeps low profile,due to the reasonable utilization of the surplus space of the array.2.A low correlation and mutual coupling MIMO(Multiple Input Multiple Output)antenna is designed.First of all,based on the idea of radiation pattern diversity,two different mushroom-like EBG structures are designed to support and stop surface wave propagation,separately,in the 4.9 GHz band.Then,they are applied to a two-element MIMO antenna,so that the diffusion path of the surface wave can be controlled effectively and the beam direction of the antenna element is changed too.Thus,the envelope correlation coefficient is improved significantly and the mutual coupling is reduced preliminarily.After that,a new defected ground structure which can change the co-polarization of the coupling field into the cross polarization is proposed,to further reduce the mutual coupling.The rectangular slots are etched on the radiating patches as well to decrease the cross-polarization level of the antenna element.As a result,the MIMO antenna works at 4.9 GHz.The envelope correlation coefficient value is lower than 0.0006.Moreover,the port isolation is maximally improved by 20 d B,compared with the reference antenna.3.To simplify the design analysis process and the complicated antenna structure of horizontally polarized or circularly polarized omnidirectional antennas,the two omnidirectional metasurface antennas adopting the analysis of characteristic mode is studied.First,by using the theory of characteristic mode,the characteristic modes and the radiation patterns of the metasurface structure are investigated in the C band.Then,by analyzing the current distribution of the characteristic mode which has the omnidirectional radiation pattern,the best excited position and feeding method are found easily.The experimental results show that the proposed horizontally polarized omnidirectional metasurface antenna has a relative wide bandwidth of 16.6%.The gain variation in the horizontal plane is less than 3 d B.The antenna profile is only 0.06?.With respect to the circularly polarized metasurface antenna,it is found that the impedance bandwidth of 16.9% is obtainned.Moreover,a good omnidirectional radiation characteristic is realized within the 3-d B axial ratio bandwidth of 11.8%,and the antenna has a low profile and simple structure as well.4.Traditional radiation pattern reconfigurable Fabry-Pérot antennas using electronically controlled devices have some drawbacks,such as the complicated structure and higher loss.To tackle these problems,by utilizing the plasticity and conductivity of liquid metal,a new Fabry-Pérot antenna based on a reconfigurable liquid metal metasurface is presented.First,several nested microfluidic channels are placed in the different zones of a phase gradient metasurface.Then,the liquid metal can be pushed into the specific zones so that the corresponding reflective phase is determined.Moreover,the designed metasurface as a partially reflective surface is used in a Fabry-Pérot antenna,and the antenna beam control can be realized well.The experimental results show that the antenna works at 5.1 GHz.Not only can the proposed antenna change the beam angle in the YOZ plane among ±20°,but also four states of the antenna beam width can be adjusted flexibly in the normal direction.Furthermore,due to no PIN diodes/varactors introduced,the reconfigurable antenna has the advantages of lower loss,simpler structure and easy fabrication.All these merits will allow better application of the proposed Fabry-Pérot antenna in future wireless communication systems. |
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