Research On Electromagnetic Shielding Method Of Frequency Selective Surface Based On Random Overlapping Ring Metallic Meshes

The electromagnetic shielding technology of optical instrument window has broad application prospects in the fields of aerospace equipment and precision optical instruments.With the increasing complexity of the electromagnetic environment and the gradual improvement of the detection accuracy,higher requirements are placed on the electromagnetic shielding technology of the optical instrument window.In order to achieve excellent frequency selection characteristics in the electromagnetic shielding band of the optical instrument window,research on optically transparent frequency selective surface(FSS)has received extensive attention.The optical transparent FSS needs to achieve high transmittance,strong electromagnetic shielding characteristics,low imaging quality effects and excellent electromagnetic transmission characteristics.At present,the existing transparent FSS is difficult to meet the above requirements.Therefore,designing a new transparent FSS to further enhance its performance is of great significance for the field of optical instrument window precision detection.In view of the above problems,this paper combines the random overlapping ring metallic mesh with the ring-shaped FSS to propose a transparent composite FSS structure to further enhance the performance of transparent FSS.In-depth theoretical and experimental research are conducted on the electromagnetic transmission characteristics,optical transmittance,optical diffraction characteristics and electromagnetic shielding characteristics of the composite FSS structure,the main research contents are as follows:In order to improve the concentrated optical diffraction energy distribution of the period structure mesh,a novel random method of parameters based on the overlapping ring mesh structure is proposed.The optical diffraction energy distribution of the random structure is solved by using the method of discretizing the random structure into a matrix.The optical transmittance is obtained according to the ratio of the light transmission area of the random mesh to the total area.The equivalent circuit method is used to model the electromagnetic shielding effectiveness of the random overlapping ring mesh.The equivalent circuit model of the square mesh is modified to obtain the equivalent circuit model suitable for the random overlapping ring mesh.The simulation results showed that the total optical transmittance of the random overlapping ring mesh is 95.96%,and the maximum of the normalized high-order diffraction energy is only 0.00157%,which greatly reduces the impact on imaging quality.The electromagnetic shielding effectiveness in the band of 12~18 GHz is higher than 23 dB.In order to realize the excellent electromagnetic transmission characteristics of the FSS structure in the millimeter wave band,the parameters of FSS structure are optimized.The impact of period parameter,inner and outer diameter parameter,annular width parameter and the thickness of the loaded substrate on the electromagnetic transmission characteristics are simulated and analyzed.The optical transmittance of the ring-shaped FSS was modeled theoretically,and the total optical transmittance was only 6.96%.The optimized ring-shaped FSS has the pass-band center frequency of 35.12 GHz,the maximum pass-band transmittance of-0.308 dB,the bandwidth at-3 dB is 10.78 GHz.In order to balance the high transmittance,strong electromagnetic shielding characteristics,low imaging quality effects and excellent electromagnetic transmission characteristics in the millimeter wave band,the random overlapping ring structure is combined with the ring-shaped FSS to form the composite FSS structure.The total optical transmittance of the composite FSS structure is 95.62%,which is increased by 88.66% compared with the ring-shaped FSS,the maximum normalized high-order diffraction energy is only 0.00158%,and the impact on the image quality is very weak.The center frequency of the composite FSS electromagnetic transmission characteristic curve is at 32.3 GHz,the maximum pass-band transmittance is-0.79 dB,and the-3 dB bandwidth is 13.4 GHz.The structure designed in this paper has been processed and tested experimentally.The experimental results showed that the optical transmittance of the random overlapping ring mesh sample is higher than 90%,and the distribution of the high order diffraction energy is uniform,which has a weak influence on the imaging quality.The electromagnetic shielding effectiveness in the band of 12~18GHz is higher than 18 dB.The optical transmittance of the FSS based on the random overlapping ring mesh is higher than 90%,the center frequency is at 31.3 GHz,the maximum pass-band transmittance is-2.6 dB,and its optical transmittance is as high as 91.6%.It improves the low optical transmittance of the traditional FSS while maintaining its original electromagnetic transmission characteristics,and its high-order diffraction energy distribution is very uniform,which has little effect on the imaging quality.The FSS based on the random overlapping ring mesh simultaneously achieves high transmittance in the optical band,strong electromagnetic shielding characteristics,excellent electromagnetic transmission characteristics,and high image quality.The research method of the random metallic mesh and optical transparent FSS have important application value in the field of precision detection optical windows and aerospace equipment.