Application Of Subwavelength Structure In Electromagnetic Stealth And Transparency

With the rapid development of science and technology,in the realization of various devices,equipment and systems,people put forward various requirements for the electromagnetic characteristics of materials.Natural materials are no longer enough to support such increasingly demanding requirements.As a result,a new artificial electromagnetic material,sub-wavelength structural material,emerged as the times require.By designing the components and structures of sub-wavelength structural elements,the singular electromagnetic characteristics that natural media do not possess can be realized,which provides a new platform for the application of materials to control electromagnetic waves.Subwavelength structures,including metamaterials,supersurfaces,photonic crystals and frequency selective surfaces,are widely used in optical physics,material engineering and other fields.Especially in the field of electromagnetic stealth and transparency,which are the key and difficult issues in national defense applications,the sub-wavelength structure also shows excellent performance.Stealth technology can greatly improve the survival and penetration ability of weapons,platforms and other targets by reducing radar cross section(RCS).At present,the main methods to reduce RCS include shape design,absorption,phase control and polarization control.Subwavelength structure can improve the performance of absorption materials and provide new methods such as phase control.The important branch of sub-wavelength structure in the application of electromagnetic transparency is the design of radome using sub-wavelength structure.Modern radomes require transparency-RCS reduction integration,i.e.transparency in one band,while RCS in other.bands is very small.From the point of view of experiment and application,this paper focuses on how to improve RCS reduction strength and bandwidth,and how to realize low loss transmission and broadband RCS reduction at the same time.The main contents and innovations are as follows:1.A p-parameter describing the matching degree of absorption material with air under complex impedance condition is proposed.Accordingly,the requirement of-10 dB reflectance on input impedance of absorbing structure is obtained.A broadband absorbing structure consisting of lossy hexagonal ring frequency selective surface,dielectric layer and metal backplane is studied and fabricated.The measured-10 dB reflectance bandwidth is 6.4-24.9 GHz and the relative bandwidth is 118.2%.2.Two kinds of broadband composite absorbing structures were studied and fabricated by combining Salisbury screen and Dallenbach layer.(1)Broadband perfect absorption structure consisting of a lossless surface,a lossless dielectric layer,a magnetic patch and a metal backplane.Because the electromagnetic parameters of magnetic materials have strong dispersion,the input impedance of the upper surface of the dielectric layer changes very smoothly with frequency by adjusting the dielectric layer.So the whole absorbing structure can achieve broadband impedance matching.The experimental results show that the reflectivity bandwidth of-20 dB is 5.3-10 GHz and the relative bandwidth is 61.4%.(2)Broadband low profile composite absorbing structure consisting of lossy hexagonal ring frequency selective surface,lossless dielectric layer,magnetic patch and metal backplane.The lossy frequency selective surface is used as the matching layer in the combination application of Salisbury screen and Dallenbach layer.The absorption bandwidth is effectively broadened.At 4-23.5 GHz,the-10 dB RCS is reduced,and the relative bandwidth is 136.8%.The thickness of the composite absorbing structure is only 6 mm,and its working bandwidth is significantly larger than that of the alisbury screen and the Dkalllenbach layer with the same thickness.3.Three kinds of lossy arrays are designed to reduce the RCS of low frequency and broadband by combining the absorption and phase control functions of the sub-wavelength structure surface with the magnetic absorption material.(1)Lossy arrays based on Lossy octagonal conductive rings.By designing the structure and material parameters of lossy array elements,the periodic arrays with different elements have the characteristics of strong absorption in the same band.At the same time,the reflection phase varies greatly in this band.Then the wideband high strength RCS reduction(-20 dB)is realized.The experimental results show that the-20 dB RCS of the lossy array reduces the bandwidth by 6.45-11.44 GHz.(2)Lossy arrays based on magnetic materials.Periodic arrays are constructed from magnetic absorbing materials.Their strong absorption characteristics are distributed in different bands,but their reflection phases vary greatly in a wide band.The combination of different primitives makes RCS reduce bandwidth greatly.The thickness of the lossy array is 8mm and the bandwidth of-10 dB RCS is reduced to 2.2-14.8 GHz.(3)Lossy arrays based on magnetic materials and lossy octagonal conductive rings.The magnetic material can achieve broadband absorption at low frequency and its thickness is small.Based on the absorption structure of lossy octagonal conductive ring,it has the characteristics of broadband absorption at high frequency.Combining the two,RCS bandwidth reduction is further broadened.The thickness of the lossy array is only 6.12 mm,and the bandwidth of-lOdB RCS is reduced to 3.15-18 GHz.4.The integrated permeability functional material(CM-FSS)with high efficiency transmission and broadband RCS reduction is realized by combining coding super-surface and frequency selective surface.By using the bandpass characteristics of frequency selective surfaces and the phase control capability of coding supersurfaces,the broadband RCS is reduced in the stopband of frequency selective surfaces,and the low loss transmission is realized in the passband of frequency selective surfaces.Using PSO optimization algorithm,the two functions are balanced.The transmittance of CM-FSS reaches the maximum value of-0.24 dB at 4.6 GHz,the transmission bandwidth of-1 dB is 4.1-5.0 GHz,and the transmission bandwidth of-15 dB RCS is reduced to 8.5-13.5 GHz.