| The metasurface composed of arranged subwavelength scatterers in a thin layer is a kind of two-dimensional artificial material that shows designed electromagnetic properties.It has been widely used as the polarization selective surface,frequency selective surface,low-radar cross section(RCS)surface,beam control surface,waveguide of surface waves,high gain antenna,and so on.The metasurface,as a new member in electromagnetism,still remains a number of problems to be solved in practical application.For example,the beam-control metasurfaces with low-efficiency are bulky in size and weight.The application of the metasurface antennas is limited by the high-coste,narrow working band and unwanted changing of the radiation pattern with the frequency.A number of methods have been proposed by researchers to solve these problms.The problems of the metasurface in practical application,however,have not been solved perfectly.It is significantly important to study new design procedures of metasurfaces to improve their performances.In this dissertation,the restriction of the metasurface on radiation and scattering has been studied to enhance its working efficiency.The cause of the restrictions has been analyzed in detail and some method have been proposed to improve the performance of the metasurface.The main contents and innovations of this dissertation are as follows:The phase gradient metasurface has been applied to controlling the direction of the reflected and transmitted plane waves.The efficiency of the anomalous reflected beam,however,is low when the reflected angle is far from the incident wave and the reflected wave is co-polarized with the incident one.The low-efficiency performance of the metasurface has been explained and the limitation of the phase gradient metasurface has been studied.With the introduction of multiple cross-polarized or co-polarized surface waves,the designed metasurface can control the reflected beam effectively when the incident and reflected waves interact with each other.By controlling the direction of the reflected wave and absorbing the incident wave,the RCS of the metasurface is reduced.For a common metasurface with low RCS,the size of the scatterers on the surface and the distance between the scatterers are far smaller than the operating wavelength.The metasurface is bulky in size and the tunable devices are difficult to be implemented.The metagrating,in which the distance between the neighboring scatterers is bigger than half of the wavelength,is an extension of the metasurface.In this dissertation,the lossy metagrating is proposed to reduce the RCS of the surface.The surface with low RCS designed based on the metagrating is lightweight and easy to implement tunable devices.The related formulas of the lossy metagrating are derived in detail and several low-RCS metasurfaces are designed.The antenna designed based on the one-order sinusoidally modulated surface has a high-gain beam.The design procedure is easy and the gain of the antenna is high.It is easy to be fabricated with good performance.The antenna,however,has high sidelobes in the visible region which reduce the efficiency of the antenna.The sinusoidally modulated metasurface with the high-order term is proposed in this dissertation.The high-order sinusoidally modulated surface can control the grating lobes.The sinusoidally modulated metasurface antenna is with high-gain and low grating lobes.The metasurface,based on the holographic theory,has been applied to designing the high-gain leaky-wave antenna.The design of the metasurface,however,is quite difficult and the designed small metasurface antenna is inefficient.A new design method of the high-gain metasurface antenna based on the characteristic mode analysis(CMA)is proposed.The relation between the surface impedance and characteristic modes of the uniform metasurface is studied and the fundamental characteristic mode of the metasurface can be stimulated effectively by adjusting the surface impedance.Then,by using multiple metasurfaces,the radiation efficiency of the high order characteristic mode is reduced.By reducing the high order mode and radiating the fundamental mode of the metasurface,a high-gain broadside antenna is obtained.A high gain broadside metasurface antenna working at C band with the relative bandwidth of 4.8% is designed and measured. |
