| How to arbitrarily control the electromagnetic(EM)scattering is a challenging research topic in the field of Electromagnetics.Traditional optical devices usually realize waves control by utilizing dynamic phase,which inevitably restricts the structure,shape and volume of the devices,resulting in high complexity of the system,difficulty in fabricating,and difficulty in realizing miniaturization and integration.The appearance of metasurfaces solves the problems and provides a powerful tools for controlling EM scattering.In microwave frequency band,it is useful and potential to use metasurfaces controlling EM wave characteristics such as phase,amplitude and polarization in military and civil fields.In military,the Virtual Shaping Technology based on metasurfaces can change the EM scattering characteristics of the target,thus misleading the detection and perception of radar.And the Anti-detection Technology based on metasurfaces can scatter the EM waves of the target in other directions,,thus reducing the signal intensity of the radar detection area.Both methods can improve the stealth and viability of military equipment by the metasurfaces.For civil use,antenna design with directional radiation,high directivity and low RCS can be realized based on the special EM properties of metasurfaces,such as anomalous reflection and polarization control.Furthermore,it is possible for realizing the low cost and small size of civil phased array antenna without phase shifter based on the metasurfaces,which provides strong support for intelligent driving and 5G communication technology.In this dissertation,the study is forced on the circuit phased-type metasurface reflectors,the special property of waves control and conformal design.Firstly,the main factors that affect the scattering and phase modulation of metasurfaces are analyzed based on the equivalent model,scattering field,electromagnetic coupling and impedance matching theory;Secondly,the array theory is innovatively applied to the curved surfaces to form a generalized array theory which can guide the scattering control of the curved metasurfaces,and the metasurfaces with the properities of scattering control is optimized by the Genetic Algorithm;Thirdly,the backscattering properity of the electrically large open-ended cavities are analyzed based on the Geometrical Optics.It provide a novel approach to suppress the backscattering of cavities by loading diffusion metasurfaces into the inner walls,which effectively realizes the broadband,wide-angle monostatic RCS Reduction of cavities.In the study of flat metasurfaces,the geometry,thickness and material parameters of metasurface units are optimized for reflection phase modulating based on the impedance matching condition and equivalent circuit method.On this basis,the Array Theory and Genetic Algorithm are used to realize different arrays and scattering of flat metasurfaces,and show good scattering control effect.For the case of single lobe scattering control,a40×40 array metasurface is used to achieve accurate and efficient directional scattering in the range of scattering angle[0°,60°];For the case of complex scattering control,broadband 3-lobe scattering control is realized,and the design is verified by experiments that the scattering is good controlled in the range of 8.7 GHz to 11.3 GHz with the fluctuation amplitude under 3 dBm.In the study of curved metasurfaces,the phase factor of non-planar structure is introduced in original array theory,extending their application to curved structures.Based on the extended array theory,curved metasurfaces with high scattering control performance is designed.In TE and TM polarization,the normal reflection of designed curved metasurface is improved from 4.6 dBm~2 and 2.8 dBm~2 to 17.8 dBm~2 and 17.2dBm~2,respectively.Considering the difference between curved surface and flat,and the difference between normal reflection and oblique reflection,the main factors raising the two differences are analyzed based on Maxwell equation and scattering field of structure.When the angle between incident and anomalous reflection increases,the matching of surface impedance becomes worse,which leads to the increase of parasitic reflection and the decrease of scattering control performance.In the application research of metasurfaces,the design idea is put forward for backward RCS reducing of electrically large open-ended cavity.By loading diffusion-type metasurfaces,the mirror reflection of angular reflective structure can be suppressed,which can effectively reduce the strong backscattering of the cavity.In X-band,simulation and experimental results show that the method has achieved good RCS reduction for different height cavity structures.The average of monostatic RCS is reduced over 10 dB in the angle range of[10°,60°].In summary,this dissertation makes a thorough study on the characteristics of EM scattering control,conformal design and the application of RCS reduction direction of super-surface.The mechanism and influencing factors of EM scattering control of metasurfaces are analyzed from the principle and phenomena,which is significant for designing flat and curved metasurface with good performance of EM waves control,and low RCS cavity. |
