The Study Of FSS Absorbers With Wide-angle Oblique Incidence Stability

As a new type of structural absorbing material,Frequency Selective Surface(FSS)absorber plays an important role in electromagnetic stealth technology.In most cases,radar waves are obliquely incident on the surface of target.Therefore,the study of FSS absorbers with oblique incidence stability has vital practical application values.This paper aims to design a FSS absorber with properties of polarization insensitivity and angular stability in the band of 4-8 GHz,which provides a new idea for the design of wide-angle oblique incidence absorbers.Methods of analyse and design absorbs under oblique incidence are introduced,including the variation of wave impedance under oblique incidence and the equivalent vector transmission line model,which provide theoretical bases for the impedance matching of FSS absorbers in transverse electric(TE)and transverse magnetic(TM)polarization modes.The Smith chart is used to analyze performances of the Salisbury screens with variable dielectric layers under normal and oblique incidence.The results are consistent with that of the full-wave simulations,indicating that the guidance from the Smith chart to the design of FSS absorbers under wide-angle oblique incidence is feasible.A polarization insensitive absorber under 45 degrees oblique incidence in 4-8 GHz is designed and fabricated.With the guidance of the Smith chart,the full-wave simulation method is used to design and optimize the asymmetric FSS unit pattern and lossy devices in TE and TM modes,so that the requirement of polarization insensitivity can be satisfied by matching impedance of different polarizations under 45 degrees oblique incidence.The resonance mechanism of the absorber is revealed by analyzing and comparing the loss and the electric field distributions of TE and TM polarizations at resonance points in different frequencies.The resonance peak in lower frequency is mainly related to the capacitance between the units,while the resonance peak in higher frequency is mainly related to the capacitance inside the unit.The equivalent circuit parameters of the unit are accurately fitted by the software,and the design system which made up of of equivalent circuit,full-wave simulation and Smith chart is realized.The test results of the experimental sample further verify the simulation conclusions.An active tunable absorber with 0-45 degrees wide-angle oblique incidence stability in 4-8 GHz is designed and fabricated.By analyzing the variation of wave impedance at oblique incidence,the domain of oblique incident angle is transformed into a specific intermediate angle value of 30 degrees.By analyzing the freuency dispersion characteristics of FSS pattern,scaling factors are introduced to improve the design of the unit pattern.Active devices are used to adjust the real and imaginary parts of the impedances of FSS absorber in order to satisfy the impedance matchings at different angles of oblique incidence.Further more,the double-sided FSS is adopted to realize independent loading of the dual tunable active devices,so as to achieve polarization insensitivity and wide-angle oblique incidence stability.The experiment results agree well with the simulations.