Research And Design Of Artificial Electromagnetic Metasurfaces For Ultra-Broadband RCS Reduction

With the continuous development of radar detection technology,various electromagnetic equipment,especially military equipment,has an increasing demand for electromagnetic stealth technology.In the increasingly complex electromagnetic environment of the battlefield,the survivability and combat capability of aircraft,ships and other equipment with low radar cross section(RCS)characteristics in a wide frequency band will be significantly improved.In recent years,metasurfaces have become a research hotspot in the field of stealth technology due to their low profile,easy conformality and flexible design.The application of the RCS-reduced metasurfaces into the aircraft,ships and other equipment can effectively improve their stealth performance.However,how to achieve RCS reduction in a wider frequency band is still an urgent problem to be solved.Antenna is an indispensable device in wireless communication system,which plays an important role in transmitting and receiving information.Reflective metasurfaces often have an adverse effect on the radiation performance of the antenna for the reduction of its RCS.Therefore,the reduction of the antenna RCS without degrading its radiation performance is also a factor to be considered when designing the metasurface.Based on the above application requirements,this thesis mainly focuses on widening the RCS reduction bandwidth of the metasurface and realizing the broadband RCS suppression out of the working band of the antenna.The main research contents of this thesis are as follows:1.An ultra-broadband metasurface with hybrid scattering cancellation and absorption is proposed for the RCS reduction.The proposed metasurface is composed of broadband polarization conversion-absorption units,which are mainly composed of three layers: lowfrequency polarization conversion-absorption layer loaded with lumped resistors,highfrequency polarization conversion layer and metal ground.The loaded lumped resistors can not only suppress the resonances generated by the low-frequency structure at high frequencies,but also absorb a part of the incident electromagnetic wave.The orthogonal arrangement of the low-frequency polarization conversion-absorption layer and the highfrequency polarization conversion layer gives rise to their orthogonal surface current distributions and thus avoids the generation of Fano resonances.In this scenario,the working bandwidths of low-frequency and high-frequency structures are well merged together.Therefore,the polarization conversion-absorption ratio(PCAR)of the polarization conversion-absorption unit in the ultra-broadband range of 2.7～19.6 GHz is greater than 0.9.The polarization conversion-absorption unit is formed into a 6×6 sub-array,and then the subarray and its mirror array are arranged in a 2×2 checkerboard-like configuration to form an RCS reduction array in the hybrid mechanism of the scattering cancellation and the absorption.The final simulation and measurement show that the proposed metasurface can achieve more than 10 dB RCS reduction in the range of 2.85～18.65 GHz with a fractional bandwidth of 146.98%.2.An ultra-broadband metasurface with a transmission window is proposed for the RCS reduction.In order to reduce the RCS of the antenna without affecting its radiation performance,the proposed metasurface can realize the function of efficient wave transmission in the passband and broadband RCS reduction in both side bands in a hybrid way of scattering cancellation and absorption.The metasurface is composed of the polarization conversion-absorption units with the frequency selective property.The proposed unit mainly consists of three parts: the low-frequency polarization conversionabsorption layer loaded with lumped resistors and an interdigital structure,high-frequency polarization conversion layer,and the second-order bandpass frequency selective surface(FSS)layer.The introduction of the interdigital structure avoids the loss of electromagnetic wave energy in the passband while keeping the polarization state unchanged.The application of the FSS with transmission zeros narrows the transition band between the passband and the polarization conversion-absorption bands.The full-wave simulation shows that for the vertically polarized incidence of the electromagnetic waves,the bandwidth of the copolarization reflection coefficient less than-10 dB covers 3.16～25.04 GHz with a fractional band of 155.18%.The transmission bandwidth for the co-polarization transmission coefficient greater than-1dB is 8.45～10 GHz,in which the minimum insert loss(IL)is 0.43 dB.The final simulation and measurement show that the proposed metasurface can transmit waves efficiently in the range of 8.45～10 GHz with a low loss,and meanwhile achieve the RCS reduction more than 10 dB in the range of 3.4～24.6 GHz.