Impedance Characteristics Regulation And Design Of Resistive Films For The Frequency Selective Surface Absorber Applications

The resistive films for frequency selective surface(FSS)absorber applications are receiving increasing attention on naval stealth due to its radar stealth and its stability to marine environment.The impedance regulation of resistive film FSS over a wide frequency range,especially in the grating lobe region,and the impedance matching of the dielectric-resistive film FSS composite absorbing structure are urgent problems to be solved in current resistive film FSS research.Therefore,this dissertation takes resistive film FSS as the research object.The interaction pattern of two-dimensional infinitely thin resistive film FSS topology on microwave response is extensively elucidated and the equivalent circuit-FSS topology parameter set is constructed.Further,the regulation of surface impendance characteristics of thick films FSS is analysed and the impedance characteristics of resistive film FSS in grating lobe region are systematically explored.These results provide theoretical guidance for the impedance matching design of composite broadband microwave absorbing structures,which is of great academic value and potential engineering application.The main research findings of this dissertation are as follows:Firstly,the electromagnetic response of two-dimensional infinitely thin resistvie film FSS in microwave frequency band and the non-grating lobe region is investigated.By clarifying the correspondence between the electromagnetic field response of the resistive film FSS and the circuit response of the lumped devices,the equivalent circuit-resistive film FSS topological parameter set is obtained and a “circuit–field” transformation design method(CFTDM)is proposed,which provides a theoretical guidance for the efficient design of resistive film FSS absorbers.Based on this,the rapid design and verification of single-layer and multilayer resistive film FSS wideband absorbers are carried out.The single-layer absorber achieves-10 d B absorption at 3.7～16.6 GHz,and the double-layer absorber further expands the-10 d B bandwidth to 1.4～18 GHz.The results of microwave reflectivity measurement are consistent with the simulation,which verifies the validity of the correspondce between topology parameters and circuit parameters and the universality of the CFTDM.Secondly,the microwave surface impedance mechanism of thick resistive film is elucidated.In the first place,an equivalent two-dimensional model of the thick resistive film is developed and its equivalent surface impedance boundary conditions are derived.The result shows that the negative imaginary part of the 2D equivalent surface impedance of the highimpedance resistive film has a crucial influence on its electromagnetic wave response and design in the microwave band.Further,the correspondence between the complex surface impedance and the DC square resistance of the thick resistive film in the microwave band is experimentally clarified,providing a method for describing and predicting the microwave response of thick resistive films based on a 2D infinitely thin resistive film FSS model.By introducing the negative imaginary part of the surface impedance,a high impedance thick resistive film absorber working in C and X bands is designed and fabricated.The total thickness of the absorber is 4.05 mm,and the-10 d B bandwidth is 4.5～13.3 GHz.The ratio corresponding to the total thickness of the absorber and the lowest operating frequency wavelength is close to the theoretical limit.The correctness of the equivalent two-dimensional surface impedance model for thick resistive films is verified through the design and measurement.The establishment of equivalent 2D surface impedance boundary of thick resistive film unified the surface impedance form of thick resistive film and 2D infinitely thin resistive film FSS,and laid a theoretical foundation for the equivalent impedance regulation of resistive film FSS and the realization of absorber.Thirdly,the microwave response mechanism of the resistive film FSS with the basic topological pattern in the grating lobe region is further revealed.Based on the calculation of equivalent impedance of resistance film FSS and the analysis of scattering field,the suppression characteristics of resistive film FSS with stable equivalent impedance in scattering field are illustrated,which provides a design guidance for suppressing and reducing the grating lobe amplitude of resistive film FSS array.Therefore,the strategy of suppressing the large period resistive film FSS is proposed.The topology of the patch pattern resistive film FSS with stable equivalent impedance is adopted,or the pattern of the ring resistive film FSS is zigzagged to achieve the stable equivalent impedance in the grating lobe region.Furthermore,a high resistive film absorber with grating lobe suppression is designed and fabricated.Compared with the same period absorbing structure without grating lobe suppression characteristics,the scattering amplitude at the angle of grating lobe is reduced by about 15 d Bsm,which verifies the grating lobe suppression effect of resistive film FSS absorber.This resistive film FSS with stable equivalent impedance breaks through the application limitation of large-period array in the broadband range and provides a new idea for the design of wide-band absorber of large-period resistive film FSS.At last,an integrated design method of dielectric-resistive film FSS composite broadband absorbing structure is put forward.The effect of dielectric loading on the input impedance and the absorption performance of the composite absorbing structure is comprehensively investigated.Under the condition of actual limited dielectric material system and thickness,the impedance characteristic regulation and impedance matching design of the composite broadband absorbing structure are carried out.Herein,an impedance inverse transformation method is proposed for impedance matching of the integrated structure.Further,an impedance modification strategy is adopted to break through the constraint that the impedance transformation form of dielectric layer is relatively simplex,which increases the freedom of design and expands the absorption bandwidth.Based on this,a multilayer dielectric-resistive film FSS composite absorbing structure operating in X and Ku bands is designed to achieve absorption performance below-10 d B in the range of 8～18 GHz,with strong absorption below-20 d B in the range of 9.1～13.2 GHz.Finally,a large-period array with grating lobe suppression characteristic is introduced to achieve the absorption of-10 d B in the range of2～18 GHz.This expands the low-frequency performance of the absorber while taking into account the radar cross section(RCS)reduction at the grating lobe of absorber.This work breaks the limit of cell period size for broadband absorbers,and might open up new avenues for the application of large-period arrays in the field of electromagnetic wave modulation.