Research On Mechanism And Design Of Ultra-wideband Wide-angle Circuit Analog Absorbers

Radar absorbers can absorb a certain proportion of incident electromagnetic waves in a specific frequency band,which is the main implementation method of radar cross section(RCS)reduction and stealth technology in military field.However,at the same time,with the development of modern radar detection technology,the absorbers need to have not only the characteristics of ultra wideband and large absorption angle,but also low profile and dual-polarization.In order to achieve the above goals,the circuit analog absorbers(CAAs)with ultra-wideband and large incident angle are taken as the research topic.This dissertation firstly studies the scattering characteristics of the absorbers for wide frequency band,establishes the equivalent analysis model and deduces the corresponding formulas.Then,by using the methods of multi-resonance and wide-angle impedance matching layer,the dissertation puts forward the design scheme and analysis method to improve the operating bandwidth and angle stability,respectively.The design,calculation,optimization,performance characterization and test methods of this kind of absorbers are finally successive studied.The main contents of this dissertation are summarized as follows:1.Based on the basic bandgap FSS,an accurate and efficient design method for resonant FSSs is proposed.Firstly,for the ideal situation without dielectric substrate or with air substrate,it is found that the metal gap of bandgap FSS will cause obvious current non-uniformity,which violates the basic assumption of traditional equivalent circuit method.Therefore,a correction term,called the effective width,is introduced into the traditional mathematical calculation model of equivalent inductance and capacitance.The reviewed mathematical expressions for inductance and capacitance are given through full wave simulation and parameter fitting.On the other hand,considering the actual model with dielectric substrate,it is found that the physical parameters of the dielectric substrate also have a great impact on the overall FSS frequency response results.Therefore,a new correction term,effective permittivity,is further introduced into the calculation of equivalent inductance and capacitance,and the final calculation formulas are given in detail.The results of simulation,calculation and measurement show that the resonance frequency and frequency response of wide frequency band are consistent in a specific application range.The design method proposed in the dissertation not only has high calculation accuracy,but also can be used to study and design much more complex resonant structures.It provides an accurate and efficient mathematical model for the analysis and design of this dissertation.2.Based on the new equivalent circuit mathematical model,an ultra-wideband single-layer circuit analog absorber with low profile and small unit size is proposed.First,the disadvantages of the two main methods to improve the absorption bandwidth of CAA are analyzed.It is found that there are dimension-parameter constraint and mutual coupling when using single-layer multi-resonant elements,which deteriorates the overall absorption.On the other hand,multi-layer structure will significantly increase the total thickness.By designing two lossy square-loop FSS arrays on the upper and lower surfaces of the dielectric substrate,respectively,the mutual restriction of element dimensions is eliminated,and the dimension freedom as well as the optimal performance is greatly improved.Besides,the quasi-single-layer structure is adopted to ensure the low profile and small unit size at the same time.The calculated,simulated and measured results verify the accuracy of the design method.Further,it is theoretically analyzed with the equivalent circuit that what is the influence of the number of resonant circuits on the absorption performance of the quasi-single-layer absorber.The bandwidth limit of the single layer structure is determined,and two feasible methods to break through the limitation are proposed.3.Based on the new equivalent circuit mathematical model proposed in the previous dissertation,a novel and effective design method for absorbers with ultra-wideband and low profile using multi layer and resonances is proposed,by converting the equivalent transmission line of dielectric substrate into corresponding parallel admittance.In view of the bandwidth limitation of single-layer absorbers,starting from the basic Salisbury absorber,the resonant FSSs and multi layers are introduced to achieve impedance matching in different operating frequency bands.Finally,an ultra wideband CAA with four resonances is realized by using the double-layer square-loop FSSs of embedded with lumped resistors.Compared with the reported results of single or double layer absorbers,the designed model not only has the largest absorption bandwidth of 171.2%,but also the overall thickness is only 0.087?_L.The calculation,simulation and test results verify the accuracy of the design method.4.Based on the new equivalent circuit mathematical model proposed in the previous dissertation,a novel method for the design of broadband wide-angle circuit analog absorber with wide-angle impedance matching layer is proposed.From the point of view of impedance matching,it is found that the deterioration of absorption performance under oblique incidence is mainly caused by the decrease of the dielectric layer's admittance introduced with the increase of incident angle.Furthermore,the impedance effect of wide-angle impedance matching layer composed of double-layer square-loop FSS under normal and oblique incidence is analyzed.It is indicated that the influence of dielectric layer can be obviously offset by the effect of the dielectric layer when the incident angle increases,so that an ultra-wideband radar absorption with±45°can be realized.The accuracy of the design method is verified by the calculation,simulation and test results.