| Since electromagnetic absorbers can effectively absorb the incident electromagnetic waves through some physical effects by converting electromagnetic wave energy into thermal energy,they play an important role in national defense and civilian fields.With the application of new radar detection technology to weapon systems,electromagnetic absorbers occupy a special strategic position in modern warfare.In addition,with the rapid development of information technology,electromagnetic stealth and interference have attracted more and more attention in the daily life.Researches on the application of high-performance absorbers for shielding or eliminating electronic interference have become a hot topic in recent years.Aimed at the strict requirments on microwave absorbers in practical engineering,such as wideband,low profile,light weight,strong strength etc,this work focuses on the high-performance absorbers,where low-profile broadband technology and dynamic tunable methods are proposed and employed for the design of the microwave absorbers.The main research includes the following aspects:Firstly,a set of segmented measurement covering the main microwave frequency band is proposed.If the test frequency is above 2GHz,the free space measurement system is proposed,which is simple and flexible.If the frequency to be tested is below 2GHz,a transverse electromagnetic wave(TEM)cell measurement is built,which has the advantages of high reliability and low cost,making up for the lack of low-frequency measurement as well.Secondly,a dynamically tunable wideband microwave absorber based on graphene and random metasurface is proposed.By stacking four layers of graphene,the tunable range of graphene sheet resistance is lowered to 380Ω/sq-80Ω/sq.In addition,by choosing twelve proper elements of metasurface and distributing them randomly,more resonant frequencies and phase responses can be achieved,thus improving the bandwidth of the microwave absorber and reducing its profile simultaneously.The equivalent circuit analysis is applied to explain the mechanism of absorption.By tuning the sheet resistance of graphene via bias voltage from 0V to 4V,the absorption of proposed absorber can be tuned from 80% to 50% in a wide bandwidth from 5 GHz to 31 GHz.The prototype of proposed absorber is fabricated and measured.The reasonable agreement between the simulated and measured results verifies the accuracy of our approach.Thirdly,two kinds of ultra-wideband microwave absorbers are proposed.Based on the principle of multilayer interference cancellation and ohmic loss,four layers of resistive films with different resistance values and different patterns are placed at certain intervals to achieve an absorption rate of more than 82%in the ultra-wideband from 0.5GHz to 18GHz.On the basis of this absorber,the second one is improved by loading a high-permittivity dielectric plate and piercing metallic via to improve the absorption performance and incident angle stability simultaneously.Finally,the two samples are processed and measured to show the wideband absorption performance of the absorber. |
