Characteristic Mode Analysis Methods And Their Equivalent Impedance Regulation Applications For Frequency Selective Surfaces

Circuit analog absorber is a structural electromagnetic absorbing material,which has the advantages of lightweight and wide absorption band,and has a great application prospect in the field of reducing the radar cross section of high-value targets.At present,the research on the performance improvement of the circuit analog absorber mainly focuses on the design of lossy frequency selective surfaces.In this dissertation,the main objects of our study are the loaded/unloaded frequency selective surfaces.In order to investigate the relationship between the modal current and electromagnetic response of the frequency selective surface and their variation with load impedance,the entire basis currents used in Periodic Moment Method(PMM)were constructed and the effect of load impedance on the modal behavior and scattering characteristic of the frequency selective surfaces was extensively studied.The relationship between the equivalent impedance of the frequency selective surface and the absorption characteristics of the absorber and the regulation method of the equivalent impedance of the frequency selective surface were explored in detail,which provide a theory instruction and a design guidance for the design of the single-layer and multi-layer circuit analog absorber.The main research results of this dissertation are as follows:At first,to extend the application range of the modal analysis method based on PMM,an approach of constructing the entire basis current using characteristic mode current was proposed,and an equivalent load resistance model was given for a load resistor.In the calculation of PMM,the basis currents are put forward to select according to the modal significance and the modal weighting coefficient.Although only the truncation rule with the modal significance greater than 0.01 can be used to select all the modes that are actually likely to be excited,only a few modes are excited under certain excitation conditions.And the use of modal weighting coefficient can further reduce the number of basis current.The scattering properties of the lossless and lossy frequency selective surfaces were calculated by the Periodic Moment Method(PMM)and Characteristic Mode Analysis(CMA)program PMM-CMA and compared with that obtained from the computational electromagnetic software FEKO,the correctness of the proposed basis current construction method was confirmed.The mode excitation for an array can be qualitatively determined by the mode excitation of a single element.This study reveals an association between the modal current and electromagnetic response of a frequency selective surface.Namely,the resonant frequency and bandwidth of a mode determine the resonant frequency and bandwidth of the array to some extent.In multi-mode system,the resonant frequency of the array is affected by the interaction between different modes.The physical mechanism of modal interaction null is that the vector superposition of modal scattering field is zero.In the study of lossy frequency selective surface,the introduction of a load resistor increases the modal significance bandwidths of the modal currents,and the contribution of the high order modes to the scattering field becomes larger at low frequencies.Therefore,more current modes need to be considered in the calculation.Secondly,the modal behaviors and scattering characteristics of the frequency selective surfaces under different load resistances and loading methods were investigated based on PMM and CMA.By inspection of modal scattering field and modal current distribution,it was found that the resonant frequency of the lossy frequency selective surface will move to higher frequency with the increase of load resistance due to the influence of high order modes,and then gradually close to a certain frequency and total reflection occurs at the frequency point.Furthermore,a design method of an electrically tunable two-state reflective/transmissive frequency selective surface was proposed.To improve the isolation of the two states,two methods were proposed around the contribution of the fundamental mode to scattering field.One method is to move the fundamental mode to a higher frequency.The other method is to reduce the bandwidth of fundamental mode.The approach of reducing the bandwidth of fundamental mode was verified by experiments,and a two-state frequency selective surface with the operating frequency of about 2.0 GHz was obtained,and the isolation is more than 10 d B.In addition,the multi-point loading method was proposed to improve the phenomenon that the resonant frequency of the lossy frequency selective surface moves to the higher frequency,which can flexibly regulate the equivalent impedance of the lossy frequency selective surface.To verify the multi-point loading method,a radar absorber with high absorptivity was designed and has a-18 d B absorption bandwidth of 1.92 GHz ～ 3.99 GHz.It is proved indirectly that the multipoint loading method can reduce the contribution of high order modes to scattering field at low frequencies and improve the phenomenon that the resonant frequency of lossy frequency selective surface moves to higher frequency.Further researches show that the resistive-film frequency selective surface is an extension of the frequency selective surface based on multipoint loading method,and the scattering properties of the frequency selective surface can be effectively regulated without obviously changing the resonant frequency of the array.Finally,to further widen the bandwidth of the circuit analog absorber,the design methods based on the multi-mode structure were proposed and applied to the design of single-layer and multi-layer absorbers.It was found that the equivalent impedance of the single-mode structure is usually strong capacitive at low frequency and strong inductive at high frequency,which can be effectively improved by using multi-mode structure.At the same time,a multi-mode absorber covering L-and S-bands was obtained.Then,the test sample was prepared by inkjet printing process,and the-10 d B absorption performance of 1.0 ～ 4.5 GHz was verified by NRL arch method.In addition,a design method of the ultra-wide band absorber based on multi-mode structure and capacitive surface was proposed.The multilayer circuit analog absorber design process and the simulation examples were also given.We finally got a circuit analog absorber with a total thickness of 36.8 mm,which has a microwave reflection less than-10 d B over a frequency band of 0.88 ～ 10.82 GHz,covering the microwave frequency bands of L-,S-and C-band,and some part of P-band and X-band.