Conformal Load-Bearing Microstrip Antennas Based On 3D Orthogonal Woven Composites

Smart materials that are rapidly developed since 1990s are such composite structures which can sense and detect external(or internal) stimuli like electricity, light and heat;can respond with active control in real or near-real time in accordance with the designed mode.Smart materials are widely used in aerospace field,such as shape-changing airplane,smart skin technology,etc.The most effective way to realize smart structures is to embed the active element into the structures like the wings and airframes,to enhance the structure efficiency.Nowadays in the world the most popular smart skin technology CLAS-conformal load-bearing antenna structures,is a combination of microstrip antennas and sandwich core composites or laminated composites.Microstrip antennas are embedded into the structures to be conformal with the airframe,which overcome the shortcomings of traditional antennas from the structure point of view,with improved anti-damage capabilities and aerodynamic properties.However,laminated composites have such drawbacks as delamination,complicate manufacturing process and uneven structures thus the structures cannot be made into conformal antennas.Therefore,it's a great trend to study and develop conformal load-bearing microstrip antennas based on 3D woven composites for next generation.They are especially important for the design of airspace crafts,hidden technology for warships and scout for public safety.Kevlar12 and E-glass fibers are selected in this study as the reinforced dielectric substrates.Two co-planar fed microstrip antennas are designed and the possibility for designing extensive antenna patch arrays has been explored.Ansoft HFSS was used to simulate the antennas,particular analytical model has been established in this paper,that the traditional copper film antennas are transformed into mesh-grid pattern woven antennas and the density of copper wire ends have been simulated to investigate the antenna performance.We have found that as the S/D ratio increases which means space between copper wires increases,the absolute VSWR value of corresponding antenna decreases—return loss increases,resonant frequency moves left—resonant frequency decreases,and radiation pattern shows that with the S/D ratio increases,the radiation efficiency,gain and directivity are greatly undermined.The mesh-grid pattern is the main reason for the discontinuity of the radiation element,thus has negative impact on the antenna performance.To investigate which part has the most serious impact,we divided the conductive components into three parts:the microstrip line,the patch and the ground.Results show that the microstrip line and the ground have minimal impact on the antenna performance that VSWR value and radiation pattern have no significant difference with the designed one,the resonant frequency has slight deviation while the patch has the most serious impact. As the S/D ratio increases,the return loss of the corresponding antenna becomes larger and the radiation efficiency,gain and directivity are greatly weakened.When the S/D ratio reaches 4.6,the antenna gain is below zero.And when S/D ratio reaches below 1.4,the VSWR value is close to 1 and the radiation pattern is almost the same as the designed copper film antenna,functions as an active antenna. Considering the application condition,we have chosen S/D=1.7 to do the experiment.In the fabrication process,the upper and the bottom layer of the 3D woven composite preform were replaced with copper wires,to fabricate the microstrip line, the patch and the ground according to the yam density of the composites,using a special method.The three parts of the radiation element were composed of high performance fibers,which were bond by Z-yams.Hence there would not be any problem with delamination subject to load or damage.Two antenna patches have been designed in this experiment,orthogonal patch and interlaced patch.In the former one warp and weft yams were aligned orthogonally,bond by Z-yarns while in the later one warp and weft yams were interlace with each other.Four coupons(warp and weft for each type of patch antennas) were consolidated by VARTM method.Antenna performance testing showed that the VSWR value were all close to 1—return loss for each type was especially low.The resonant frequencies were slightly deviated from the designed central frequency 1.5 GHz.Both orthogonal antennas had similar radiation pattern with the designed one while both interlaced ones had large back and side lobes in the radiation pattern.This is because the existence of the curvature of copper wires in interlaced coupons lowered the reflective efficiency of the ground.Part of the electromagnetic wave did not be reflected to the most intense direction.Gains for four coupons were all below zero.This may be due to the resin on top of the antennas,which increases the loss of the antenna,or due to the round cross section of the copper wire.Besides,water absorption of aramid fibers would influence the dielectric properties of the antenna and the designed parameters still have some deviation from the applicable one.Mechanical properties of the microstrip antennas based on 3D woven composites in this experiment have been tested,including the following four aspects: tensile,bending,compression and impact.The results of tensile,bending and compression show that the microstrip antennas based on 3D woven composites can achieve the mechanical properties of the normal 3D woven composites:tensile strength of 3DIMA is 334 MPa(compared with the baseline—normal 3D woven composites without copper ground 343 MPa),Young's modulus 20.6 GPa(baseline 16.5 GPa),bending strenghth 238 MPa(baseline 347 MPa),bending modulus 14.9 GPa(baseline 16.9 GPa),compression strength 164 MPa(baseline 165 MPa), compression modulus 10.4 GPa(baseline 12.5 GPa).As for impact testing,data do not show the brittle nature of the damage process of the materials.The impact strength is only 6.29 kJ/m2,however the data diffraction is as high as 67%.This is because the existence of the conductive ground interferes the failure mechanism of the composites,that when impacted under relatively high velocity(compared with the quasi-static damage in bending test),the bottom layer copper wires which are stretched cannot afford the high load as the high performance fibers do,thus greatly undermines the impact strength and energy absorption of the composites.We conclude that the 3DIMA developed in this study can almost achieve the same level of the traditional copper film antennas and at the same time,the mechanical properties of 3DIMA can present the strength of normal 3D orthogonal woven composites,which are superior to dielectric substrates for traditional microstrip antennas—laminated composites.