| Frequency Selective Surface(FSS)is a kind of periodic spatial filtering structure,and as it interacts with the electromagnetic(EM)wave which shows high-pass,low-pass,band-pass or bandstop response characteristics,and therefore it could be widely used in EM products,such as radar radome,antenna system,dichroic prism,etc.Previous studies were mainly focused on rigid plates or composites,and the forming methods were corresponding printed circuit board(PCB),laser etching,digital carved milling methods,rarely involving the research of soft materials.The related properties of textile materials,like dielectric,antistatic,EM shielding properties,etc,have been numerously reported,while the researchers neglect the fact that textiles can be a medium in the EM field to regulate and control EM transmission.Yarns are made of abundant low linear density as well as controllable fibers,and fabrics consist of a large number of yarns in different systems,and therefore the yarns and fabrics have typical periodic structure.Also,the diameter dimension of different fiber assemblies matches well with diverse EM wave wavelength.Besides,a variety of local metallization processes become mature gradually,making the design and development of EM functional textiles the development become possible.Based on the research status of traditional FSS and referring to existing research ideas and analysis methods,the thesis carried out a systemic research on fabric based FSS,mainly including the investigation of response characteristics especially the band-pass characteristics,experimental analysis of influencing factors,The exploration of various preparation techniques,design of narrowband fabric-based FSS,evaluation of response characteristics under certain bending.Specifically,the research contents are focused on the following aspects:First,based on the empirical formula proposed by Munk,the rough design of dimension parameters was proposed with regard to 10GHz(X-band radar)using the cross-shaped and Jerusalem-shaped units.And the precise experimental parameters were determined through computer-based carving experiment and the experimental scheme was proposed.Then a series of samples were fabricated and tested,and the influencing rules of various parameters on the response characteristics of fabric based FSSs were obtained through comparative analysis.Measured results showed that two testing conditions,namely anechoic chamber and non-anechoic chamber,have small influence;the fabric based FSSs with two complementary conductive units showed opposite but not completely complementary frequency response characteristics,revealing that two kinds of structures need to be designed separately;dimension parameters have large influences because of the change of equivalent impedance of structure unit,for example,increase of unit space would decrease the bandwidth;as the equivalent dielectric constant of fabric substrates increased,the resonance frequency would decrease;the conductivity of common metal would produce the similar frequency response characteristics;the frequency response characteristics of cross-shaped and Jerusalem-shaped fabric based FSSs showed good stability to the EM wave polarization modes,because two kinds of units could coincide with the original units as they rotate 90 degrees,meaning that the array characteristics had no differences in two perpendicular directions;as for the stability of frequency response characteristics to the EM incident angle,the Jerusalem-shaped fabric based FSS was superior to the cross-shaped fabric based FSS.Second,the feasibility of preparing fabric based FSS using different technics was explored.The preparing methods of fabric based FSSs were aggregated,and representative methods were listed.On the basis of computer-based carving process,other processes,like silk-printing process,computer embroidery process,coating printing process,etc,were adopted to prepare fabric based FSSs,the measured verified that these methods were effective,but the processing precision,sample size,processing cost,etc,may have certain limitations,for example,the prepared samples through small computer embroidery machine could not meet the test requirement,and in the coating printing process,the sample precision was not good enough.In the actual product development process,the more mature cloth hot stamping process could be adopted.Various methods could be adopted to prepare fabric based FSS and they were very convenient,which offered the technical guarantee for the relevant topics.Third,on the basis of analyzing the influencing factors of frequency response characteristics,the design of narrow-band fabric based FSSs were carried out,and the bandwidth level was evaluated.The design principles were concluded,such as the grating lobe grating lobe,equivalent circuit(EC)model,etc.Two models were simulated respectively using two different algorithms,and the results showed very small differences,verifying the validity of design methods and reasonability of simplifying models.Taking square-loop-shaped fabric based FSS as an example,the corresponding samples were prepared and tested,and the measured results were compared with the simulation as well as EC model results,further verifying the feasibility of design procedures.Then,six typical radar wave frequencies were selected as target resonance frequencies,and the overall experiment was designed,comprehensively considering the influencing factors,such as the resonant frequency,the shape of FSS unit and the coincidence as the unit rotate 90o,etc,and the samples were prepared according to the design parameters.Testing results showed that the bandwidth level was approximately equal to the bandwidth level of existing research results domestic and overseas,and at certain frequency,the narrow-band characteristics were more ideal.For example,the U-shaped unit was adopted for experiment design,and measured results showed the resonant frequency indeed appeared at 1GHz under the TE polarization mode,and the bandwidth(the threshold of-0.5d B)is 0.19 GHz,basically same with the 0.17 GHz level in the literature.The cross-shaped unit was adopted for experiment design,and the measured resonant frequency indeed appeared near 10 GHz and the bandwidth value(the threshold of-3d B)was 1.66 GHz,smaller than the 2.4GHz level in the literature.Last,based on the research results and conclusions of planar fabric based FSSs,the frequency response characteristics of the curved fabric based FSSs were studied.The curved surfaces could be cylindrical,tapered,spherical,etc,and the cylindrical fabric based FSS was selected as an example to perform simulation process and experimental analysis.The differences between curved and planar fabric based FSS in the simulation model,simulstion settings as well as simulation results were investigated.The transmission coefficients of simulation models consisting different unit numbers were calculated,and the analysis of radar cross section(RCS)characteristics were added.The experiment was designed based on the the unit pattern and size parameters in the previous narrow-band design,and the samples were fabricated and tested.Comparison of the measured and simulation results verified the validity of the design method,and further the influences of bending curvature,bending direction and unit pattern were studied.The results showed that: as the bending curvature increased,the transmittance of EM wave reduced and the transmission coefficient of patch curved fabric based FSS were more stable than aperture type;compared with the circumferential bending,the axial bending had a greater effect on the frequency response characteristics of aperture curved fabric based FSS;the frequency response characteristics of the cross-shaped and Jerusalemshaped fabric based FSSs were more stabilized when bended,however,the frequency response characteristics of Y-shaped fabric based FSS,which was non-coincident after 90o rotation,showed bigger fluctuation.at the same curving effect. |
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