| Radar Cross Section(RCS),as an important indicator for evaluating stealth capability,is used to measure the ability of a target to scatter electromagnetic waves.RCS reduction techniques mainly include shape design,stealth material loading,passive stealth,and active stealth.Among them,the use of stealth material loading has become the most effective means of achieving RCS reduction,especially when the target shape has been determined,and has therefore received extensive research attention."In recent years,metamaterials have been favored by researchers due to their low cost and easy controllability.Among them,meta-surfaces based on Orbital Angular Momentum(OAM),with their unique phase structure and mutually orthogonal modes have become one of the hotspots in the field of meta-surfaces research." The spiral phase front and phase singularity characteristics of OAM are expected to be applied in the field of electromagnetic scattering control.Therefore,the thesis carries out research on low RCS metasurface design based on OAM.Based on the goal of low RCS metasurface design based on OAM,the thesis carries out a series of research works,including OAM metasurface design based on patch unit phase gradient surface,OAM metasurface design based on polarization converter,and low RCS metasurface design based on OAM singularity cancellation,etc.,to achieve extremely low RCS metasurface structures,explore the electromagnetic transmission mechanism of low RCS metasurfaces based on OAM,and the effectiveness of the design method was verified through experiments.The main research content and innovation points of this thesis are as follows:(1)A design method for OAM metasurfaces based on square patch structures is proposed.The design principles of two types of OAM metasurfaces,namely the ordinary type and the focused type,are elaborated,and the design process based on the two types of vortex metasurfaces are summarized.The ordinary type OAM metasurface is designed and the RCS reduction effect under different mode numbers is explored.Under the vertical incidence of electromagnetic waves,the RCS reduction is over 30 d B and it is insensitive to polarization.Based on the calculation formula of convergent OAM,the compensating phase required for forming vortex waves on the metasurface is obtained.The unit arrangement is optimized to obtain the convergent type OAM metasurface,which achieves good RCS reduction effects of over 30 d B under TE and TM polarization.(2)A design method for OAM metasurfaces based on polarization conversion unit structure is proposed.A square ring resonant unit structure is designed,which utilizes the characteristics of the polarization structure to reduce the difficulty of satisfying the phase conditions required for generating OAM metasurfaces and improve the quality of reflected vortex beams.The effects of ordinary and convergent OAM metasurfaces composed of the proposed unit structure on RCS reduction under different orders are studied.The generation of vortex beams and RCS reduction of the metasurfaces are simulated and tested for two mode numbers.The results show that the above structure achieves RCS reduction of over 10 d B under HH and VV polarization waves,with a reduction bandwidth of 2 GHz and a maximum reduction of over 25 d B.(3)A design method for low-scattering metasurfaces based on singularity cancellation is proposed.By using patch units and polarization conversion structures,vortex beams with positive and negative singularities are introduced,and the combination structure enables the total reflected beam to achieve electromagnetic scattering reduction through singularity cancellation.Under vertical incidence of different polarized waves,both metasurfaces exhibit great RCS reduction characteristics,with a maximum reduction value of-39.74 d B.Compared with a single OAM metasurface,the RCS reduction value increases by 6 d B at the reduction extreme. |
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