| Metasurface is a kind of two-dimensional metamaterial composed of artificially designed periodic or aperiodic subwavelength structure.The metasurface has the advantages of flexible regulation the polarization,phase and amplitude of electromagnetic wave,showing extra electromagnetic phenomena that natural materials do not have.It provides a new research idea for developing of new stealth materials.At present,some achievements have been made in radar cross section(RCS)reduction by using metasurfaces.However,how to design metasurfaces with dual wideband,dynamic regulation and ultra wideband RCS reduction characteristics in microwave band remains to be further studied.Therefore,in order to achieve excellent RCS reduction stealth performance,this work focuses on the RCS reduction performance of rigid polarization conversion coding metasurface(PCCM),opaque flexible PCCM,transparent flexible metasurface and composite transparent flexible coding metasurface.The models of metasurface elements with specific function are constructed,and the electromagnetic transmission characteristics of metasurface element are analyzed by transmission line theory.The optimization algorithm is used to arrange the metasurface elements in space to realize the coding metasurface with different frequency response characteristics.RCS reduction performances of rigid PCCM,opaque flexible PCCM,transparent flexible metasurface and composite transparent flexible coding metasurface are studied,and the mechanisms of dual wideband,dynamical regulation and ultra wideband RCS reduction are revealed.The samples are prepared and RCS experimental tests are carried out to verify the effectiveness of the proposed designs.The main research of this paper are as follows:A rigid polarization conversion coding unit model with specific phase frequency characteristic is built by CST software,and the influence of model parameters on polarization conversion ratio(PCR)is discussed.Combined with spatial layout optimization,the dual-band RCS reduction characteristics of rigid PCCM under different spatial layout are studied.The internal relationship between PCR and RCS reduction is established.RCS reduction characteristics of diffuse rigid PCCM in dual-band under different incident angles and different polarized waves are deeply analyzed,and the mechanism of dual-band RCS reduction is revealed.The samples are prepared by printed circuit board process and RCS experiments are carried out in the microwave anechoic chamber.The construction of polarization conversion coding unit of dual-band and high PCR is the basis for PCCM to effectively suppress dual-band RCS.Numerical simulation results show that the diffuse rigid PCCM has mirror reflection function in 14.4-15.6 GHz and can achieve more than 10 d B dual wideband RCS reduction in9.9-14.1 GHz and 15.6-20.6 GHz under x-polarized wave normal incidence,and the maximum monostatic RCS reduction is 30.6 d B.The diffuse rigid PCCM not only has polarization insensitivity,but also has dual-band RCS suppression in the range of 45°.Numerical simulation and experimental results show that the reflective rigid PCCM has the ability of frequencyselective RCS suppression,and its dual-band RCS reduction comes from the phase cancellation interference in the dual-band through the regulation of phase dispersion.To further study the dual-band RCS reduction performance of opaque flexible PCCM based on phase cancellation,the rigid dielectric substrate material polytetrafluoroethylene of polarization conversion coding unit is replaced with polyimide and polyvinyl chloride.According to the Pancharatnam Berry(PB)phase principle,the spatial arrangement of opaque flexible PCCM with diffuse type is determined by using an anisotropic element and genetic algorithm.The dual-band RCS reduction characteristics under the flat plate,different curvature radius and different model parameters are emphatically investigated.Sample is prepared by flexible printed circuit process to verify its dual-band RCS reduction performance.The experimental results show that diffuse opaque flexible PCCM can achieve more than 10 d B RCS reduction in 9.47-12.68 GHz and14.67-19.18 GHz in the planar case under y-polarized plane wave normal incidence,and the relative bandwidth is 28.98% and 26.65%,respectively.The proposed metasurface is conformally bent on metallic cylinder with the height of 240 mm and the curvature radius of 45 mm.10 d B RCS reduction is obtained in 9.61-12.18 GHz and 14.54-18.49 GHz,and the relative bandwidth is 23.59% and 23.92%,respectively.The experimental results are basically consistent with the numerical simulation,which indicates that the metasurface has the ability of dual-band RCS suppression under the planar and conformal cases.The performance of dual-band RCS reduction of diffuse opaque flexible PCCM increases with the increase of curvature radius of conformal cylinder under the fixed size,and the dual-band RCS reduction bandwidth can only be adjusted by model parameters.In order to realize the dynamic regulation of RCS reduction and visual visibility,the novel graphene-based absorbing transparent flexible metasurface in microwave band is built.The dynamic absorbing characteristics are analyzed in detail by using the transmission line model.The effects of graphene sheet resistance,absorbing element parameters and incident angle on the absorbing characteristics are studied.Moreover,RCS dynamic regulation characteristics of transparent flexible metasurface are explored under the planar and conformal cases.The research shows that the transparent flexible metasurface presents dynamically regulated microwave absorption characteristics by changing the graphene sheet resistance.The transparent flexible metasurface whether in the planar or conformal case shows more than 10 d B dynamic regulated characteristics of RCS reduction in dual-band(6.66 GHz and 17.57 GHz),wideband(7.49-17.66 GHz)and single band(12.18 GHz)when the graphene sheet resistance is 70 Ω/sq,240 Ω/sq and 350 Ω/sq,respectively.The dynamic regulation characteristics are attributed to the strong interference at the resonant frequency or in the wide frequency band,which effectively captures the incident electromagnetic waves in the transparent flexible metasurface,and the incident wave energies are dissipated with high ohmic loss.Aiming at the problem of how to obtain ultra wideband RCS reduction,the composite transparent flexible coding metasurface is constructed by integrating phase cancellation and absorpting wave.The functions of transparency and flexibility are also considered.How to effectively integrate the two mechanisms and how to build the metasurface unit cells are investigated.RCS reduction characteristics of composite transparent flexible coding metasurface are deeply analyzed under the planar and conformal cases.In addition,RCS reduction performances under different polarization and oblique incidence are investigated.The sample is prepared by laser etching process and the tests of RCS and transmittance are carried out.The theoretical results show that most of the incident wave energies(70.64%)are absorbed and a small part of the energies(20.09%)produces phase cancellation.The numerical simulation results show that the relative bandwidth of greater than 10 d B RCS reduction is 102.96% and96.93% under the planar and conformal cases,respectively.The ultra wideband RCS reduction is due to the composite mechanism of phase cancellation and absorpting wave,which can not only regulate the angular distribution of scattered wave energy,but also convert the incident wave energy into heat energy through ohmic loss.Therefore,the metasurface can reduce the scattered field intensity in almost all directions and has better RCS reduction performance;The composite transparent flexible coding metasurface possesses the polarization insensitivity.Moreover,RCS reduction of the metasurface can be maintained within 60°. |
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