Research On Design Theory And Method Of Low-profile Metamaterial Absorber With Wide-angle Characteristic

Absorber technique has been widely used in military and civilian fields.A perfect absorber should have characteristics of broadband,angle insensitivity,polarization insensitivity,and low profile.Due to the characteristics of high design freedom,simple structure,low profile and high absorption rate of the absorber based on the metamaterial design concept,the metamaterial absorber has the potential to realize perfect absorber.Metamaterial absorbers have developed rapidly in the past decade.At present,the research on the bandwidth of metamaterial absorbers is relatively mature,but the research on the theory and design methods of wide-angle metamaterial absorbers is still relatively lacking.So,new design theories and methods are urgently needed.This dissertation has carried out in-depth research on low profile metamaterial absorbers with wide-angle absorbption properties.The main contents are as follows:Firstly,design method of wide-angle metamaterial absorbers based on the principle of antenna reciprocity is studied.Starting from the antenna's scattering field characteristics and receiving characteristics,the feasibility of using a wide-angle scanning array for wide-angle metamaterial absorber design is theoretically analyzed.Two absorbers with wide-angle incidence based on the principle of antenna reciprocity are designed.Firstly,based on the principle of mirror image,a wide-beam antenna unit with a dipole parallel to the artificial magnetic conductor(AMC)is designed.Then,a suitable lumped resistor is loaded at the feed port.Finally,a wide angle absorber with more than80%absorption in the 80°incident range is achieved.The profile of the absorber is only0.085?,where?represents the wavelength of the center operating frequency.Compared with other wide-angle absorber,the proposed absorber has a wider absoption angle.In addition,based on the concept of tightly coupled antennas and wide-angle matching layers,a butterfly-shaped tightly coupled unit is designed.Then,a suitable lumped resistor is loaded at the feeding point and a medium-air-medium matching layer is loaded on top of the unit.The operating frequency band of the absorber is 1.97-9.42 GHz under normal incidence and 2.64-6.4 GHz under 60°incidence.Although the operating frequency band gets narrower with the increase of the incident angle,the absorber still realized the performance of wide-band and wide-angle absorbption properties.The profile of the absorber is only 0.14?_L,where?_L represents the wavelength of the lowest operating frequency.Compared with the other planar wide-band and wide-angle absorbers,the structure of the absorber based on tightly coupled concept is much simple.The absorber shows better performance in terms of absorption angle,bandwidth,and profile.Secondly,design theory and method of wide-angle metamaterial absorbers based on the equivalent medium theory are studied.Starting from the equivalent medium theory,the reflection coefficients of the dielectric block backed with a perfect conducting plate under the illumination of TE(Transverse Electric)wave and TM(Transverse Magnetic)wave are derived in detail.The relationship among the reflection coefficient,the thickness of the dielectric block,the equivalent electromagnetic parameters and the angle of incidence is analyzed.Based on the theoretical analysis,we can find the value of the equivalent electromagnetic parameters that the absorber can achieve wide-angle and high-efficiency absorbing performance.Then,the corresponding metamaterials that meet the requirement can realize the design of wide-angle absorbers.Based on the theoretical analysis,a wide-angle metamaterial absorber with a double-sided split resonant ring loaded with resistors and capacitors is proposed.At present,the parameters of the unit structure are needed to be adjusted to realize the metamaterials with specific equivalent electromagnetic parameters,and their adjustment degrees of freedom are relatively low.In order to increase the degree of freedom in the adjustment of the equivalent electromagnetic parameters of the metamaterial unit,a lumped resistor is introduced to accurately adjust the equivalent electromagnetic parameters.Meanwhile,a capacitor is also introduced for adjusting the resonant frequency of the metamaterial and the miniaturization of the unit.The resistors and capacitors are loaded in parallel at the opening of the double-sided split resonant ring.The theoretical analysis results,simulation results,and test results are basically consistent.The absorber can maintain an absorption rate of more than 90%in the incident angle range from-70°to+70°and also achieve more than 80%absoption rate when the incident angle is 75°.The absorber shows better performance in term of absoption angle compared with other wide-agnle absorbers based on special structures.Morover,the center frequency does not significantly shift.The profile of the absorber is only 0.0148?,where?represents the wavelength of the center operating frequency.The design of a wide-angle and low profile metamaterial absorber with good center frequency stability is realized.The absorber can also show the same wide-angle absorption characterictics at the new resonant frequency while only the loaded capacitor is changed.Based on the unit of the metamaterial absorber,through combining two unit planes with different capacitance values,the design of the dual-frequency bandwidth angular metamaterial absorber is realized.Furthermore,based on the unit,a design scheme of frequency-tunable wide-angle metamaterial absober is given.Thirdly,design method of ultra-wideband and wide-angle metamaterial absorber based on 3D printing is presented.Compared with two-dimensional planar structure metamaterial absorbers,three-dimensional absorbers have more design freedom due to the third dimension.Thus,3D metamaterial absorbers have great potential in the field of wide-bandwidth and wide-angle absorbers.With the continuous maturity of 3D printing technology,combined with the high dispersion and high loss characteristics of water in the microwave frequency band,a rudder-shaped water-based metamaterial absorber is designed.The absorber element is a rudder-shaped micro-cavity container filled with water.When the cavity of the cube is filled with water,it can operate from 6.9 to 90 GHz with 90%absorptivity.For TE wave incidence,it can achieve 90%absorptivity within±40°.It can achieve 90%absorptivity within±60°for TM waves.The profile of the absorber is only 0.218?_L,where?_L represents the wavelength of the lowest operating frequency.The water-based metamaterial absorber can achieve an ultra-wide bandwidth and wide incident angle wave absorption performance.Compared with present water-based absorber,the proposed water-based absorber shows better performances in terms of bandwidth and absorption angle.