| As a passive reflector,Luneburg sphere reflector has the advantages of high reflection efficiency and wide angle response range.It is commonly used to enhance the radar cross section(RCS)of targets or build radar targets with specific scattering characteristics.The traditional Luneburg sphere reflector is composed of a dielectric sphere with gradually changing dielectric constant and a spherical coronal metal reflector.Its reflection echo characteristics for electromagnetic waves are in the full frequency range,and once produced and finalized,the RCS characteristics are immutable,greatly limiting the flexibility of building a target.This article studies the method of using frequency and amplitude controllability of Frequency Selective Surface(FSS)to achieve RCS adjustability of Luneburg sphere reflectors,in order to expand the scattering control ability and application range of Luneburg sphere reflectors.The main research content and innovation points are as follows:1)Research on RCS frequency adjustable technology of Luneburg spherical reflectorPropose a scheme to use FSS instead of the metal reflector of the Luneburg sphere reflector to achieve RCS frequency gating.Adopting conformal miniaturized FSS as the reflector of the Luneburg sphere reflector,a new Luneburg sphere reflector is formed.The simulation has verified the RCS frequency selection characteristics of the new Luneburg sphere reflector.Its RCS value at the band stop FSS resonance frequency point is approximately equal to the RCS value of the traditional Luneburg sphere reflector.The RCS within the FSS passband decreases with the decrease of FSS reflection coefficient,achieving a Luneburg sphere reflector with frequency selection characteristics.In the research process of frequency adjustable Luneburg sphere reflectors,two conformal arrangement methods of surface FSS,spherical conformal arrangement and square arrangement,were proposed.The effects of curvature radius,size,and oblique incidence angle on the reflection characteristics of surface FSS were studied in both cases,and the performance changes of conformal FSS relative to infinite plane FSS were analyzed.Comprehensive analysis shows that considering the stability of oblique incidence,polarization stability,and frequency selection characteristics,the scattering performance of the Luneburg sphere reflector using spherical conformal arrangement FSS is better than that using square conformal arrangement FSS.2)Research on RCS amplitude adjustable technology of Luneburg spherical reflectorA scheme is proposed to use active frequency selective surface(AFSS)absorbers instead of the metal base plate of the hemispherical Luneburg sphere reflector to regulate RCS amplitude.To reduce the weight and profile of the Luneburg sphere reflector,a hemispherical Luneburg sphere is used to replace the spherical Luneburg sphere in traditional Luneburg sphere reflectors;Replacing the metal reflective base plate of the hemispherical Luneburg reflector with an AFSS absorber to achieve the regulation of the echo amplitude of the hemispherical Luneburg reflector,that is,using the two working states of absorption and reflection of the AFSS absorber to make the hemispherical Luneburg reflector base plate exhibit absorption and reflection characteristics of incident electromagnetic wave energy,respectively.The research results indicate that the RCS difference of the low profile hemispherical Luneburg sphere reflector in the absorption/reflection state is above 8d B when the elevation direction is 20°～90°;In the azimuth direction of 65°～65°,it has a wide angle response of 130°,and the RCS difference in the absorption/reflection state also exceeds 8d B.This verifies that the structure of the Luneburg sphere reflector can freely switch between the absorption and reflection states,achieving amplitude regulation of the RCS of the Luneburg sphere reflector. |
