Artificial Electromagnetic Material And Its Applications In Electromagnetic Stealth Technology

Artificial electromagnetic material(or metamaterial),which is composed of periodically arranged unit cells,is an important discovery in recent years.The size of its unit cells is much smaller than the incident wavelength,and thus,can be viewed as macroscopic electromagnetic material.Due to its ability to realize arbitrary permittivity and permeability,it has attracted wide attention of researchers at home and abroad.Artificial electromagnetic materials with wide band characteristic can be applied to a wide range of microwave devices or systems.And the low profile,easy conformal artificial electromagnetic surface(or metasurface)provides a new approach to the stealth platform design due to its ability of flexible electromagnetic wave control.In this dissertation,we mainly investigate the wide band artificial electromagnetic surface,and its applications in stealth technology.The main contents of this dissertation are as follows:1.The operating principle of the artificial electromagnetic material is explored.Based on the theory of electrodynamics,the equivalent constitutive parameters of infinitely long wire array and split ring resonator array are calculated analytically.And the influence of geometry parameters on equivalent constitutive parameters is discussed.The NRW method is presented to retrieve the equivalent constitutive parameters of artificial electromagnetic material,and the algorithm is verified by calculating the constitutive parameters of a F4 B medium block as well as the equivalent permittivity of infinitely long wire array.2.A variety of artificial electromagnetic surfaces with wide band characteristics are designed.By using the multi-resonance technology,the fan shaped and disc shaped artificial electromagnetic surface,with wide negative permeability frequency band characteristic,are designed.The relative frequency band of each structure is 30% and 50%,respectively.The Operating principle of each structure is analyzed,and the equivalent circuit models of each structure are established.The equivalent projection method(for fan shaped structure)and infinite partition method(for disc structure)are proposed to explain the wideband operating characteristic.The calculation formulas of the equivalent magnetic resonance frequency and the electric resonance frequency of each structure are derived.The influence of geometry parameters on electromagnetic response of each structure is discussed.On this basis,the connected disc shaped artificial electromagnetic surface is investigated.Its permittivity and permeability are proved to be negative simultaneously.The electromagnetic response characteristics of this structure are analyzed.The calculation formulas of its plasma frequency are de-rived,and the relationship between the plasma frequency and the geometry parameters is discussed as well.3.The artificial electromagnetic surface is applied to the stealth platform design.The low profile absorbing surface based on artificial electromagnetic material is investigated.First,several widely known absorbing surfaces are introduced.The physical mechanism of absorbing surface is investigated.An equivalent LC circuit model is established for the qualitative analysis,and an equivalent resonant cavity model is proposed to provide quasi-quantitative analysis.The calculation formulas of absorption peak frequency for both models are deduced.On this basis,an equivalent transmission line model is proposed to realize accurate and quantitative analysis,where the patch layer and the substrate of absorbing surface are viewed as lumped circuit and equivalent microwave transmission line,respectively.Then,the accurate calculation formulas of absorption rate and absorption peak frequency of absorbing surface are derived.Finally,due to the low efficiency of traditional exhaust design method,interpolation method is applied and a semi-analytical numerical method for fast absorbing surface design is proposed.The semi-analytical numerical method can dramtically reduce the design time when comparing with the traditional exhaust design method.A slotted disk shaped absorbing surface is designed based on this method.4.New RCS reduction method is explored and the idea of low RCS random artificial electromagnetic surface is proposed.By uniformly scattering the incident energy,the scattered field energy density in each direction tends to be uniform,which will significantly reduce the threat from both monostatic and bistatic radar system.According to the antenna array theory,the equivalent model of the random artificial electromagnetic surface is established,and the theoretical calculation formulas of the scattered field distribution are derived.The influence of geometry parameters on the scattered field is studied as well.A random artificial electromagnetic surface with 19 dB maximum backward RCS reduction is designed,and an optimized artificial electromagnetic surface with 25 dB maximum backward RCS reduction is studied as well.On this basis,a wide band,low RCS random artificial electromagnetic surface,which is not sensitive to polarization and incident angle of incoming wave,is designed by arranging several sub arrays with different operating frequencies.A random artificial electromagnetic surface sample is fabricated and a RCS measurement is performed.The simulated and measured results show that the structure provide over 10 dB backward RCS reduction from 7GHz to 12GHz(with a relative bandwidth of 52.63%),and the maximum RCS reduction is more than 28 dB.5.Chessboard structures,with anomalous scattering properties,are investigated to pro-vide RCS reduction characteristic.First,the operating principle of the traditional chess board structure is analyzed.The influence of structure parameters on the scattering field and RCS reduction properties is discussed as well.However,the traditional chessboard structure can be easily detected by bistatic radar systems.Thus a random chessboard structure is proposed to solve this problem.The theoretical formulas of scattering field are derived.And the influence of geometry parameters on the RCS Reduction feature is studied as well.A random chessboard structure with 21 dB maximum backward RCS reduction is designed,fabricated and measured.By using wide band unit cell and AMC sub array,a wide band random chessboard structure with 20 dB maximum backward RCS reduction in the frequency range of 10.46 GHz and 19.36GHz(relative bandwidth is 59.69%)is investigated.It is shown that this structure is insensitive to the incident angle and polarization,and the specular reflection can be effectively suppressed as well.