Designs Of Amplitude-phase Modulated Metasurfaces And Their Applications In Antenna Radiation Manipulations

Metamaterials are a novel type of artificial composite materials,which are usually composed of periodic or non-periodic arrangement of subwavelength particles in 3D space.They are designed to flexibly manipulate the propagation of electromagnetic waves.One of the most classic engineering applications for metamaterials is antenna design.As an example,effective medium metamaterials are commonly used to construct high-performance lens antennas.Metasurfaces can be viewed as 2D version of metamaterials.They have excellent capability in the modulation of phase and amplitude of electromagnetic waves.Therefore,transmission-type,reflection-type and leaky-wave antenna arrays based on metasurfaces are widely used to achieve predesigned farfield radiation characteristics,such as high-directivity beam generation,vortex beam generation,beam shaping,multiple-beams radiation and so on.Due to their negligible thickness,small volume,light weight,low loss and relative convenience in fabrication and integration,metasurface based antannas have more advantages and are more promising in engineering field.In 2014,Prof.Tie Jun Cui proposed the concept of digital coding metamaterials,which provides new perspectives in the analysis and design of metasurfaces.Digital coding description and modulation methods not only simplify design process,reducing the difficulty in realizing multi-functional metasurfaces,but also introduce information science to the research of metasurfaces,building a bridge between physical and digital world.The development of digital coding and programmable metasurfaces further broadens applications of metamaterials in novel communication,imaging and radar systems.Phase modulation is commonly used to manipulate radiation from the metasurfaces.With further research,to flexibly realize more complicated farfield pattern,amplitude-and-phase modulation has attracted more and more attention.Most available schemes are based on C-shaped structure with polarization conversion.Due to the coupling between amplitude and phase responses,independent modulation of co-polarized amplitude and phase still needs more investigation.The topic of this thesis focuses on the design of different types of metasurfaces for co-polarized amplitude and phase modulation.Working principle and design process for leaky-wave-type,transmission-type and reflection-type metasurfaces are discussed respectively,with an emphasis on their applications in the generation of multiple beams of different power allocation.The results verify the capability of metasurface to achieve complicated radiation design and its potential in antenna applications.The main contents and contributions of this thesis are summarized as follows:1)Classic antenna array theory is systematically reviewed.The far field of infinitesimal dipole is first discussed.It is used as basic building block to deduce array factors for continuous source antenna and antenna array according to vector superposition principle.Several methods adopted in metasurface designs to manipulate antenna radiation are also briefly introduced,such as far field superposition for different patterns,convolution theorem and antenna synthesis based on numerical optimization.Furthermore,most common metasurface aperture amplitude and phase distributions and their corresponding far field characteristics are summarized in details.A metasurface to simultaneously realize retro-reflection under two incident angles is used as an example to illustrate the generalized design process for actual metasurface antennas.2)A one-dimensional amplitude-modulated leaky-wave antenna is proposed to achieve dual beams radiation with different power allocation.According to the general design process of metasurfaces,an approach for aperture amplitude modulation is introduced based on leaky-wave mechanism.Combined with convolution theorem,it is used to realize a specially designed aperture amplitude distribution in cosine-wave-loaded form.By varying the modulation depth coefficient and with the help of simulation optimization,dual beams radiation of different power is achieved simply by aperture amplitude modulation.The directions for and power ratio between the two beams can be designed independently.3)A transmission-type 2-bit amplitude and 2-bit phase digital-coding metasurface is proposed to achieve multiple beams radiation with arbitrary power allocation.The aperture of the metasurface is obtained by vector superposition of sub-patterns with different complex weights.Each sub-pattern corresponds to a single beam radiation in predesigned direction.The aperture is further discretized according to 2-bit amplitude and 2-bit phase and realized by digital-coding unit cells.By dedicately choosing sub-patterns and varying amplitude weight,the directions and power ratio of multiple beams can be arbitrarily manipulated.Furthermore,a difference coefficient is introduced to optimize phase weight,which effectively alleviates the degradation of farfield performance caused by amplitude and phase discretization of actual unit cells.This enables accurate design of power ratio under fewer bits coding scheme.Two metasurface antenna prototypes with dual beams of dirrerent radiation angles and power allocation is designed,simulated and measured to verify the feasibility of the method.4)An anisotropic reflection-type metasurface unit cell is proposed to indepedently modulates both amplitude and phase reponses of orthogonal linear-polarized waves.It is based on a variation of the cross-shaped structure.Lumped resistors of different values are loaded on two metal arms and geometric parameters are adjusted,which not only dissipates power of incident waves with ohmic losses,but also provides varied surface impedances.Therefore,complex reflection coefficients for two orthogonal linear-polarized waves can be modulated independently and anisotropically.Base on the unit cell,three types of dual-functional metasurfaces are designed and verified.Under the illumiation of orthogonally-polarized incident EM waves,they can achieve single beam radiation with different power ratio,vortex beam radiation with different orders and power ratio,and RCS reduction for specific polarization,respectively.Compared to traditional anisotropic phase-modulated metasurfaces,the proposed scheme are more flexible in designing farfield radiation patterns,laying a foundation for the realization of more complicated multi-fuctional metasurface antennas.