Design Of Broadband Periodic Absorbing Structures And Analysis Of The Electromagnetic Coupling Characteristics

Absorbing material is a kind of important functional material that can effectively absorb and dissipate electromagnetic energy that incident in it.It is widely used in low observable stealth technology,anti-metal isolation of RFID antenna,microwave anechoic chamber and electromagnetic pollution protection.However,the traditional absorbing materials,such as Salisbury screen,Jaumann absorber and absorbing coating,have the disadvantages of narrow absorbing band,large thickness and high density.And some new absorbing structures that based on metamaterials,such as perfect metamaterial absorber that composed of electric ring resonators and cut wires,and absorbing honeycomb structures that fabricated by the latest 3D printing technology,also have the similar disadvantages.These problems can not meet the requirements of broadband,thin and lightweight in the development of modern electronic information technology.In order to solve the above problems,this thesis takes the periodic unit cell absorber structure as the research object,and establishes the equivalent lumped parameter circuit model and the multi-reflection interference analysis model by analyzing the electromagnetic field distribution and energy loss state in the structure.The light polymethacrylimide rigid foam board or the honeycomb core material is used as the substrate,while the resistive sheet is used as the loss material.From the point of view of structure thickness,equivalent electromagnetic parameters and interlayer coupling,the geometric parameters of the unit cells and the constituent materials are optimized to achieve wide-band,light-weight and high-efficiency absorption of the structure.The main work and innovations are as follows:?1?An inequality-based method for selecting the proper branch of the real part of the refractive index of equivalent electromagnetic parameters is proposed.Based on the traditional parameter inversion algorithm and the relationship among equivalent refractive index,equivalent intrinsic impedance and equivalent electrical parameters,an inequality satisfying passive medium is derived to select the real part of equivalent refractive index.Compared with the commonly used Taylor series expansion or Kramers-Kronig?K-K?integral method,the proposed method is less difficult to calculate and simpler to select branches.?2?A broadband absorber with non-uniform periodic is proposed.For planar unit absorbing structure with uniform period,it is found that different resonant points need different sheet resistance for impedance matching.According to this fact,the design of planar unit absorbing structure with non-uniform period is proposed.With the same thickness,the-10dB?absorptivity A=90%?absorption bandwidth that from 4.5GHz to13GHz of the proposed structure is increased by 42%compared with the uniform one with absorption bandwidth from 6GHz to 12GHz and sheet resistance of R_sq2=170?/?.It can effectively improve the absorption performance of the structure in C-band.?3?The near-field coupling effect between the layers of ultra-thin structures is proposed,and the functional relationship between the thickness and frequency shift of structures is established.In order to realize thinner structure and lower frequency absorbing,the planar periodic absorbing structure is optimized.By comparing the results of CST simulation with those of Matlab calculation by multi-reflection interference model,it is found that the near-field coupling effect between layers can make the absorption peak red shift,which is beneficial to the design of low-frequency absorption structures.?4?It is found that the total reflection in planar structures can be suppressed and the absorbing performance of planar structures can be improved at the same time by the design of periodic absorbing structures with 3D cube unit cells.In view of the disadvantage that the total reflection will occur in the planar periodic absorbing structure with lossless substrate,a loss element is introduced into the substrate through a cube unit cell or a graphical absorbing honeycomb.In this situation,the total reflection can be effectively suppressed,and the structure's absorptivity can be improved?the low-frequency absorptivity can be improved by 20%,and the absorption bandwidth can be broadened by 43.6%?.In a summary,the absorbing mechanism of the periodic absorbing structure and the idea of broadband design has been deeply studied in this work.The method of choosing the proper branch of the real part of the refractive index during the process of equivalent electromagnetic parameters inversion is simplified.The recursive expression and transmission coefficients of the multi-reflection interference model for multilayer structures are improved.An effective method to suppress the total reflection of planar structures is proposed.It has guiding significance for the design of broadband,light weight and high efficiency absorbing structures.