| The ideal smart materials should consist of structure, function, control and information. The transmission of information is the most important issue, it is similar to human beings'eyes and ears. Therefore, the critical problem of an ideal smart materials is the transmission and reception. However, in order to solve this problem, smart materails should be combined with antennas. Microstrip antennas is widely used in many fields. Besides of its excellent performance as an antennas, it also has the best comformal ability. Thus, it has better hidden ability and relability.Fiber reinforced composites are generally used as the substrate of most smart materials and microstrip antenans. 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.E-glass fibers and PTFE matrix are selected in this study as the reinforced dielectric substrates. One co-planar fed microstrip antennas is designed. 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 3 demensional woven antennas to investigate the antenna performance. We have found that epoxy resin and vinyl ester resin will provide good mechanical properties, but both of them have high dielectric loss which will dramatically decrease the gain of 3 demensional woven antennas. However, PTFE, which has very low dielectric loss (0.0004), will largely improve the efficiency of antennas and increase the gain of antennas. Also, because of the good mechanical and anti-delamination properties of 3 demensional woven fabric, 3 demesional woven antennas with PTFE matrix will have reasonable mechanical properties.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 yarn 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-yarns. Hence there would not be any problem with delamination subject to load or damage. In this experiment, three samples were soaked in PTFE solution for one time and another two samples were soaked in PTFE for several times before consolidation. Finally, using high temperature/ pressure vulcanizing machine and oven to sinter PTFE composite. The preforms which were soaked into PTFE solution for one time have relatively low resin content and the ones which soaked into PTFE solution for several times have relatively high resin content. Antenna performance testing showed that antennas with PTFE matrix had similar radiation pattern with the designed one and the resonant frequencies were slightly deviated from the designed central frequency 1.5 GHz. The antennas which had low resin content will increase holes in composites and result in high dielectric loss. And the VSWR value is large than 2 and high return loss. Meanwhile, the gain of those antennas were -0.3,-0.4 and -0.5, respectively. However, the antennas which had high resin content means there are less holes in the composite and the dielcetric properties are similar to the designed model. The VSWR is from 1.3-1.7 and return loss is relatively low which accord with the requirement of traditional microstrip antennas. Moreover, the gains of them were 0.6 and 1.2, respectively. Also, anti-impact property of 3D woven microstrip antenna was tested by an impactor to investigate whether the performance of antennas will be changed after impact. From the result of this test, the gain and radiation pattern have not be influenced by outside impact. That means 3D woven microstrip antenna has good anti-impact property.Mechanical properties of the microstrip antennas based on PTFE 3D woven composites in this experiment have been tested, including the following four aspects:bending, compression and impact. The results of bending and compression show that 3D woven PTFE composite based microstrip antennas developed in this study have relatively low mechanical properties in terms of bending and compression strength. But it can almost achieve the same level of the traditional PTFE laminated composites. The bending strength is 27.75 MPa; bending modulus is 4615 MPa, compression strength is 13.22 MPa; compression modulus is 3126 MPa. As for impact testing, data do not show the brittle nature of the damage process of the materials. The impact strength is 10.5071J. The failure mode of the 3D woven PTFE composites is totally different from other thermosetting composites. It will not damage even the deformation is very large while the thermosetting composites will crack at a low level deformation. Also, once the applied force was canceled, PTFE composites will immediately recover to its initial state. |
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