Research On Low-frequency Broadband Radar Absorbing Metamaterials Based On Lumped Elements

Due to their exotic modulation properties for the amplitude and phase of the electromagnetic waves,metamaterial absorbers have attracted the attention of many researchers since they are proposed.Besides,considering the advantages of low cost,easy conformal,and flexible integration,they have been widely applied in the field of microwave stealth.However,due to the thickness limitation of the Rozanov limit,the current research is mainly focused on the high-frequency range.Meanwhile,to satisfy the requirements of the increasingly complex electromagnetic environment,it is inevitable to improve the focus on low-frequency,broadband,and wide-angle absorbers.Moreover,most researchers mainly take traditional design principles such as the equivalent circuit model and transmission line theory.To further easily design absorbers and understand their working mechanism,novel design methods need to be explored.Based on the traditional metamaterial cells,the thesis investigates the low-frequency absorbers from three viewpoints:novel design methods,ultra-broadband,and wide-angle performance,and the detailed contents and innovations as follows:(1)A novel design method for low-frequency broadband absorbers is proposed based on the characteristic modes theory.The working mechanism of the absorber is elaborated by mode excitation,and the detailed design flow is also summarized.First,a low-frequency broadband absorber loaded with an inner-convex square ring impedance surface is proposed,which achieves a 90%absorptivity in the range of 0.8?2.95 GHz excited by single-mode,with a fractional bandwidth of 114.5%and the total thickness is only 0.0826?_L(?_L is the wavelength at lowest operating frequency).Next,the influence of resistance on the mode significance(MS)bandwidth is explored by loading the lumped resistors,and the relationship between MS bandwidth and absorption bandwidth is also investigated.A square ring absorber loaded with resistors achieves a high absorptivity of over 90%in the range of 1.44?6.32GHz and agrees well with the experimental results.The absorber exhibits excellent performance with a fractional bandwidth of 127.3%and the total thickness is only 0.084?_L.(2)To further extend the absorption range,a double layer ultra-broadband absorber is proposed based on the principle of high-order modes excitation.The equivalent circuit model(ECM)of the absorber is established,as well as the reflection coefficient and normalized impedance distribution are in good agreement with the full-wave simulation.It is further demonstrated by experiments.The absorber achieves a high absorptivity of over 90%ranging from 1.4 to 17.31 GHz,with a fractional bandwidth of 170%and the thickness is only 0.097?_L.(3)Based on equivalent dipole far-field complementary theory,a low-frequency ultra-broadband absorber is proposed with an excellent oblique performance,which consists of horizontally and vertically placed dipole cells.The corresponding mirror-symmetric structure is presented and further optimized.Eventually,the absorber shows an excellent normal incidence performance for TE and TM polarization waves,which achieves a high absorptivity of over 90%in the range of 1.07?10.2 GHz with a fractional bandwidth of 162%and the total thickness is only 0.0946?_L.At oblique incidence,the maximum angle of incidence is up to 75°for TM polarization waves and 50°for TE polarization waves.Equivalent circuit models for TE and TM polarization at a specific incident angle are also established,and the corresponding element parameters are given and agree well with the simulation results.