| With the continuous development of anti-stealth technology under system confrontation,the combat process of future aircraft will face the battlefield threat environment of a wide frequency spectrum and wide angular domain.Therefore,it is urgent to establish a material candidate system that can satisfy the wide-band and wide-angle absorption characteristics.Honeycomb absorbers have attracted widespread attention due to their excellent electromagnetic properties and structural strength.Frequency selective surfaces(FSS)are often used to improve the low-frequency performance of absorbing materials due to their free impedance control properties.Hence,this dissertation focuses on the frequency selective surface,the honeycomb absorber,and their composite structure.Aiming at the problem of poor dispersion characteristics of the effective electromagnetic parameters of the dielectric absorbing honeycomb,a variety of impedance control methods for the honeycomb absorber are explored.From normal incidence to oblique incidence to grazing incidence of electromagnetic waves,a variety of composite structures are designed and fabricated in order to comprehensively improve their electromagnetic stealth performance,which has practical significance and academic value.The main research results and innovations are as follows:Firstly,starting from the simplest single-layer honeycomb absorber,the honeycomb absorber in the target frequency band is equivalent to a layer of a homogeneous medium by using the effective medium theory to reduce the calculation cost and improve design efficiency.Then,the specular scattering and non-specular scattering suppression mechanisms of the honeycomb absorber are analyzed respectively.For specular scattering,an important conclusion is drawn by solving the reflectivity map and impedance formula: the input impedance of the honeycomb absorber will approach its characteristic impedance with the increase of frequency,and the characteristic impedance is difficult to match with the air impedance.Therefore,the single-layer homogeneous honeycomb can only achieve narrowband absorption.In terms of traveling wave scattering suppression,the derivation of the dispersion equation proves that the honeycomb absorber has the effect of traveling wave attenuation.However,only by carefully designing the effective electromagnetic parameters and thickness of the honeycomb absorber can achieve the attenuation of traveling wave scattering in the target frequency band.Second,a grading impedance honeycomb system-level design framework is constructed to better achieve broadband specular scattering reduction for target structures under normal incidence.With the increase in the number of honeycomb layers,the impedance transition between the air and the metal plate is smoother,and the absorbing performance is improved.However,the price paid to achieve excellent performance is the increase in structural thickness and weight,which conflicts with the thin and light characteristics of the absorber.Therefore,an innovative broadband composite strategy of FSS and absorbing honeycomb is proposed,and three composite design ideas are developed.The first composite idea is FSS composite with patterned absorbing honeycomb,which not only expands the low frequency absorbing bandwidth but also significantly reduces the weight of the structure,the measurement results show that the composite structure with a thickness of 12 mm has an absorption bandwidth of-10 d B.is 2.89～18.0 GHz(fractional bandwidth of 144.7%).The second composite idea is FSS composite with an absorbing honeycomb dielectric layer,which not only improves the absorbing performance but also breaks through the bottleneck of low dielectric constant material preparation by using the absorbing honeycomb as a low dielectric matching layer.The measurement results show that the composite structure with a thickness of 15 mm has an absorption bandwidth of-10 d B is 3.1～15.0 GHz(fractional bandwidth of 131.5%).The third composite idea is FSS composite with area-coated absorbing honeycomb,which achieves the optimal absorption performance with a quasi-single layer structure,the measurement results show that the composite structure with a thickness of 17 mm has an absorption bandwidth of-10 d B.is 2.0～15.5 GHz(fractional bandwidth of 154.3%).Compared with the grading honeycomb absorber,the FSS/absorbing honeycomb composite structure not only achieves better absorbing performance but also maintains a thin and light structure.Thirdly,the electromagnetic properties of the composite structure under oblique incidence are explored,and the dielectric compensation is used to improve the incidence/polarization angle sensitivity of the FSS absorber.Considering the upper medium and lower medium of FSS respectively,two new design concepts of dual-section impedance matching and lossy absorbing honeycomb complex refraction angle are proposed.Relying on these two concepts,two kinds of absorbing structures are designed and prepared.The-10 d B absorbing bandwidth of the dual-section impedance matching absorber under normal incidence is 2.3～13.3 GHz,the absorption bandwidth under TE 45° incidence is 2.5～4 GHz,and the absorption bandwidth under TM 45° incidence is 3.1～14 GHz.For the lossy honeycomb complex refraction angle absorbing structure,the-10 d B absorbing bandwidth under normal incidence is 0.88～4.83 GHz,the absorption bandwidth under TE 45° incidence is 0.89～6.00 GHz,and the absorption bandwidth under TM 45° incidence is 0.90～6.00 GHz.The good electromagnetic properties also demonstrate the applicability of the two new design concepts,which also provide a more general design method for oblique incidence absorbers.Finally,the RCS performance of metal plate and semi-cylindrical curved surface structures in wide-band and wide-angle is studied systematically.Two multifunctional composite absorbing structures are proposed for metal plates and semi-cylindrical curved surfaces.The structure can not only reduce the specular scattering under normal incidence by virtue of broadband wave absorption properties but also attenuate the traveling wave scattering under grazing incidence by using the current loss effect of the honeycomb absorber.For the metal plate,after loading the composite structure the reduced RCS bandwidth of 10 d B under normal incidence is 0.8 to 4.0 GHz,and the reduced bandwidth of 10 d B under 20° grazing incidence is 0.8 to 4.4 GHz.For a semi-cylindrical curved surface,a 10 d B RCS reduction can be achieved in the 0.6 to 4.5 GHz bandwidth at normal incidence after loading the composite structure,and an approximate 10 d B RCS reduction can be achieved in the 1.5 to 4.5 GHz bandwidth at 20° grazing incidence.This kind of multi-functional composite structure integrating scattering suppression,bearing,and conformality provides a new idea for radar stealth technology. |
