Research On Design Of Tunable Planar Metamaterial Wave Absorber

With the rapid development of information technology,more and more electronic devices have profoundly changed the lifestyles and played an important role in various fields such as military,industry production and agriculture.Although electromagnetic equipment with a large coverage area provide great facilitation,the resulting electromagnetic radiation and electromagnetic interference are very harmful to the safety of biological and electronic devices.Generally,metamaterial absorbers（MMAs）are used in the case of electromagnetic wave forward projection,which is not suitable for planar microwave circuits.Therefore,in this thesis,the study of Ka band metamaterial absorbing structures suitable for planar circuits is proposed,which expand the application scenarios of MMAs and is conducive to further research on the application of MMAs in microwave circuits.Firstly,a MMA with an outer square inner circle unit cell is proposed.The electromagnetic field distribution is observed,and the broadband absorption performance of the MMA is enhanced by combination of adjacent resonance peaks.The tunable effect is achieved by loading an electrorheological fluid whose dielectric constant can vary with voltage.As the voltage increases,the upper cut-off frequency fluctuates slightly and the lower cut-off frequency red-shifts from 31.92 GHz to 20.72 GHz.We can illustrate the mechanism of broadband absorption in more detail by analyzing the electromagnetic field distributions under different voltages.On this basis,the effects of the thickness of the electrorheological fluid layer and the metamaterial layer on the absorption performance of the absorber are further discussed.The greater the thickness of the electrorheological fluid,the absorbing effect is better and increasing the thickness of the metal pattern,the absorptivity curve has almost no change.Secondly,two frequency-tunable MMA based on vanadium dioxide aiming at the problem of insufficient tenability of the electrorheological fluid MMA are proposed,which are single square VO₂ MMA（SSVMMA）and four corners VO₂ MMA（FCVMMA）.The simulation results show that changing the conductivity of VO₂ can change the absorption peak intensity of SSVMMA.Using the phase transition characteristics of VO₂,the four corners VO₂ MMA has 16 states with flexible tenability.The effects of increasing the square length of VO₂ and the length of the metal square on the absorption performance of the MMA are discussed respectively.When increasing VO₂ size,the absorption peak becomes narrower,and five different combinations are needed to cover Ka band.When the copper block is enlarged,four combinations are required to achieve Ka band coverage.Considering the test requirements,a layer of heat insulation dielectric is added between the first and second layers,and the frequency band with absorption rate more than 90%basically covers20～40 GHz.Finally,two frequency tunable MMA based on strontium titanate（STO）are proposed,namely single sqare STO MMA（SSSMMA）and outer square inner circle STO MMA（OSICSMMA）.The center frequency can be adjusted by controlling the dielectric constant of STO through temperature.The SSSMMA with absorption rate over 90%basically covered 20～45 GHz.The absorption bandwidth of OSICSMMA above 87%is18.72 GHz,covering the Ka band.Changing the temperature,it has good frequency tunability and works best in the proposed MMAs.Next,we discuss the effect of changing the structural parameters on the absorption rate of MMA.When the size of the resonator is reduced,the resonant frequency is shifted slightly towards lower frequencies.When the thickness of STO increases,the frequency is slightly to low frequency.