Ferrite Based Metamaterial Absorber With Tunable Absorption In Wide-range And Its Bandwidth Extension

In modern information society,microwave absorbers(MAs)have vital applications in various fields,such as electromagnetic(EM)shielding,antenna,and stealth technologyfor military purpose.The widespread existence of various and complicated EM wavesources requires the MAs to often work in a broad frequency range.Thus varieties of MAs have been so far developed.The ways to broaden the absorption bandwidth of MAs become one of the hot spots in MAs field.Now,those containing carbonyl iron powders demonstrate remarkable advantages of thin thickness,broadband absorption due to the large magnetic permeability.However,they still show inferior absorption in the low frequency range,while their permeability is reaching a ceil set by the Snoek's limit.Recently emerging metamaterial absorbers(MMAs)can obtain an absorption peak in low frequency by appropriate structure design,but the MMAs often show a narrow absorption bandwidth because of resonance absorbing mechanisms.To improve the absorption bandwidth of the MMAs,various metastructures have been developed.Nevertheless,the performances in low frequency of the MMAs are still poor by considering the thickness and bandwidth.In this paper,we use the tunable MMAs to realize a wide-range absorption in low frequency,because the absorption peak frequency of the tunable MMA can be tuned by external stimulus.However,existing tunable MMAs have too narrow tunable bandwidth,especially in GHz range,the tunable bandwidths are usually no more than 2 GHz.Thus,aferrite with broadband tunable resonance frequency under external magnetic field is selected as tuning medium so as to provide a wideband tunability,andcombined with appropriate metastructure to finally achieve a wideband tunable MMA with near-perfect absorption.On the basis of the previous work,we combine the tunable MMA and high-frequency MAs to obtain a broadband microwave MMA.Main research contents carried out are as follows:First of all,a metamaterial absorber with a near-perfectabsorption peaktunable magnetically in an ultra-wide frequency range from 0.62 to 5.74 GHz is proposed by combining a ferrite withmetal strip arrays.In our design,aferrite with broadband tunable resonance frequency under external magnetic field is selected as tuning medium so as to provide a wideband tunability.The metastructure of metal strip arrays suppress the polarization conversion brought by the ferrite and enhance the absorption to finally achieve a wideband tunable MMA with near-perfect absorption.The results showed that there are two absorption peaks,one of the peaks is from the loss of the ferrite during the diffracted waves propagating,and another is from the interference of the diffracted waves on the two sides of the metal strip.The two mechanisms are independent of each other,and their corresponding absorption peaks can be controlled by the metal strips of gap and width,respectively.Secondly,on the basis of the previous work,we remove the ferrites between the metal strips,and fill in high-frequency MAs,such as carbonyl iron powders(CIP),sheet resistance and so on,to realize an ultra-broadband microwave MMA.Taking CIP as an example,the as-designed MMA at a total thickness of 1.5 mm shows an absorption of above 0.85 in the whole frequency range of 7.5-18 GHz and a near-perfect absorption peak shifting in a frequency range of 0.62-5.74 GHz by modulating external magnetic field.By analyzed the field maps in high frequency and the influence of structural sizes on the absorption performance,we find that the absorption mechanism is closely related to the interference of the diffracted waves on the two sides of the metal strip.Thus,the high frequency absorption is related to high-frequency absorbers and the width of metal strips.The results showed that we can obtain a MMA at thin thickness with an excellent absorption via combined with MAs possessed excellent absorption inhigh-frequency.