Research On Ultra-low Profile VHF Broadband Absorbers

Modern warfare develops into a fast-paced,high-intensity,three-dimensional technological war,which the confrontation between radar stealth technology represented by metamaterial absorbers and radar detection technology becomes the commanding height of the war.The metamaterial absorber realizes the frequency selection characteristics through the frequency selective surface(FSS),and absorbs the electromagnetic energy through the lossy medium or lumped resistance,so as to realize the radar stealth by reducing the radar cross section(RCS)of target.The development of modern radar detection technology requires that the absorber not only has the characteristics of ultra-wideband and high absorption rate,but also has the ultra-low profile and polarization stability.The working performance of traditional absorbers is limited by the thickness of their profile,so it is difficult to guarantee good absorbing performance in VHF band.Therefore,exploring the design method of broadband ultralow profile absorber becomes an important trend in the development of metamaterials.This thesis explores the method to achieve broadband ultra-low profile absorption based on the equivalent model.The main contents of the thesis are summarized as follows:1.The basic theories of absorber are summarized,including the impedance matching,the propagation of electromagnetic wave in lossy medium,the equivalent model and the definition of RCS.Impedance matching theory and electromagnetic attenuation are the basis for the absorbing function of the metamaterial absorber.The analog circuit is a commonly used method to design absorber.The RCS is an important parameter which can describe the absorbing ability of an absorber,the smaller RCS of an object,the more difficult it is detected by the radar system.2.Based on the limitation relationship between the broadband and the low-profile of structure,a design method of low-profile passive absorber which is working in the VHF band is proposed.Firstly,the equivalent model is used to study the electromagnetic characteristics of the single-ring absorber,with the influence of medium is analyzed in detail.And it verifies that the radar absorber working in given frequency band has the mutual restriction between the broad operating band and low-profile of the structure,which the wider operating band,the higher of the structure profile.Then,a design method of three-layer low-profile passive absorber is proposed on the basis of the single-ring absorber,which effectively reduces the profile height of the structure while achieves the dual-polarization broadband absorbing function.The similarity between the calculation results of the analog circuit and the simulation results of the CST software verifies the effectiveness of the equivalent model and the design method.3.Based on the research of the passive absorber,a design method of ultra-low profile broadband absorber using negative impedance convertor(NIC)circuits is proposed.First of all,using analog circuit confirms the metal ground and the dielectric layer are the main reasons that limit the profile height of the metamaterial absorber.Since the equivalent model of metal ground and dielectric layer can be approximated as a specific inductance under the ultra-low profile design,choosing an appropriate negative inductance can offset their impact,which can realize the ultra-low profile absorbing function.Then,using ADS software simulates the NIC circuits that can change the inductance into negative inductance and designing the ultra-low profile absorber by the NIC circuits.The absorber has the absorption bandwidth of 55.4%,with only 0.008? profile thickness.Finally,a1×10 absorbing structure sample is fabricated for testing.The calculation results and experiment results are in good agreement in the operating band,which verifies the accuracy of the method.