| With the rapid development of electronic technology,high-technology and informatization have become important features of the modern warfare.In order to deal with the complex battlefield in the future,and improve the competitiveness and viability of the military hardware,all countries in the world have regarded the stealth technology as a core of national defense.Stealth technology has been widely used in aerospace,warships,missiles,tanks and other military areas,and has achieved significant effects in practical combat.Radar cross section(RCS)is an important parameter used to measure the electromagnetic scattering ability of the target.The RCS reduction technique can effectively reduce the scattering of the target and reduce the detection distance of the enemy radar,thus realizing the purpose of stealth.Currently,the RCS of the stealth platform itself has been well controlled,however the RCS of the antenna system loaded in the platform has become a prominent source to the overall RCS.Therefore the stealth of the antenna has become a key factor in determining the target stealth performance.The antenna is a special scattering source,and its basic role is to emit and receive radiated electromagnetic waves.The necessary prerequisite for realizing antenna stealth is to ensure the normal radiation work of the antenna.Therefore,conventional RCS reduction methods such as changing the shape,radar absorbing materials is impossible to effectively solve the problem of the antenna stealth.So far,the contradiction between antenna scattering and radiation is still the biggest challenge for the stealth of an antenna.How to maintain and further improve the antenna's bandwidth,gain and other radiation performances in the design of a low RCS antenna is of great value.Based on the theory of antenna scattering,this thesis proposes new methods for the design of low RCS antenna,taking into account the simultaneous improvement of antenna radiation and RCS characteristics.The main contents are as follows:1.A low RCS wideband circularly polarized antenna based on the ground defect structure(DGS)and quasi-fractal structure(QFS)is proposed.By grooving the radiating floor of the circularly polarized microstrip antenna,four DGSs are obtained at the center and eight H-shaped QFSs are obtained around the perimeter.Changing the current path on the floor can effectively widen the antenna impedance bandwidth and the axial ratio bandwidth,while changing the appearance and area of the floor can reduce the out-of-band RCS of the antenna.Compared with the traditional circularly polarized microstrip antenna,the 3d B axial ratio bandwidth of the proposed antenna is significantly increased from 2.444-2.458 GHz(0.57%)to 2.435-2.470 GHz(1.43%),and the gain is not reduced.Meanwhile,the proposed antenna can achieve good RCS reduction in a wide frequency range of 3-10 GHz.2.A new arrangement of metamaterial absorber(MA)and its loading technology in a circular patch microstrip monopole antenna are proposed.The absorber can simultaneously participate in the radiation and scattering of the antenna.First,a broadband three-layer MA is designed to make its absorbing band covering the antenna's working band.In order to flexibly match the circular patch in the antenna,unlike the traditional periodical arrangement,the MA cell is bent and arranged along the ring.The design and simulation method of the ring grid MA structure are given.The ring MA can maintain its original absorbing properties.A low RCS broadband low profile microstrip monopole antenna is obtained by integrating the ring MA structure with the circular patch antenna.For radiation characteristics,the entire ring MA can be seen as a radiator within the antenna operating band,producing a cone pattern.For the scattering characteristics,MA can effectively absorb the normal incident electromagnetic wave to achieve the goal of reducing the in-band RCS.3.A broadband low-profile antenna based on polarization conversion metamaterial(PCM)is proposed,which can also obtain in-band and out-of-band low RCS characteristics.First,a modification of the checkerboard structure of the PCM is designed by removing the upper patches of the PCM at the four corners.The proposed cross structure can solve the problem that the RCS reduction band is limited by the polarization conversion band of PCM.The results show that compared with the checkerboard structure(6-9.3 GHz,52.4%),the RCS reduction band(6-15 GHz,85.7%)of the new cross structure is significantly improved.Finally,the PCM-based antenna is obtained by employing this cross PCM structure,the microstrip antenna and the PCM share a radiating floor.Compared with the reference antenna,the designed antenna can produce a wide impedance match(6.05-7.52 GHz,relative bandwidth of 21.7%)and a stable gain.The RCS is reduced within 5.8-18.0 GHz(102.5%).4.A low-RCS circular-polarized F-P antenna based on a multi-functional metamaterial coating is proposed.The coating unit can independently control the amplitudes and phases of the reflection coefficient and the transmission coefficient in two orthogonal directions,achieving three functions of circular polarization conversion characteristic,partial reflection characteristic,and microwave absorption characteristic simultaneously.In the operating band,the whole coating acts as a line-circular polarization converter,which converts the linearly polarized wave from the feed into circular polarized radiati on wave;The bottomside of the coating is a partially reflective surface(PRS).A Fabry-Perot resonance cavity is constituted by the PRS and the anisotropic HIS around the linear polarized antenna.The HIS structure is used to compensate for the phase difference between the orthogonal polarizations of the PRS.The antenna gain is improved by multiple reflections of the electromagnetic wave;The top-side of the coating is the absorption layer(AS)with the resistive element s,which can effectively absorb the incident electromagnetic wave energy to achieve low RCS characteristics.The 3 d B axial ratio bandwidth of the proposed antenna is 10.63-10.75 GHz.The antenna gain is improved throughout the entire operating band,and the gain is increased by 3.2 d B at the central frequency.The antenna RCS can be reduced significantly over a wide band(4-13 GHz). |
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