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1.INTRODUCTIONCircularly-polarized antenna has been widely used in satellite communication fields and Global Navigation Satellite System due to its good polarization matching characteristic[1][2]. Microstrip antenna and waveguide slot antenna are two kinds of common forms among them[3]. By comparison, waveguide slot antenna is more suitable in high frequency and high gain demand situations, microstrip antenna is the most common because of its low cost and low profile[4]. However, conventional circularly-polarized microstrip antenna is limited by its bandwidth and axial ratio characteristic. As a result, many researches have focused on widening its working bandwidth and improving polarization performance. There are some new forms of feeding structure and radiation patch recently. For example, decreasing the Q value of the feeding line by using slot coupling is an effective way to widen its bandwidth[5]. Besides, many new kinds of radiation patch have also emerged endlessly[6][7]. Based on the theory and research findings above, an H-shaped slot-fed double-layer circularly-polarized microstrip antenna sub-array is proposed in this paper. The simulated bandwidth is wider than conventional patch. 10.3dBi gain is obtained at the design frequency. 2.DESIGN OF THE ANTENNA SUB-ARRAYThe view of the double-layer microstrip antenna sub-array is shown in figure 1. There are four layers. Corner-cut patches are placed on the first and second layers. They radiate the power from feeding structure to free space. H-shaped slot is on the third layer, and the bottom of the array is microstrip line. The size of the array is 103.2mm×103.2mm×15.43mm. 2.1Feeding lineConventional microstrip antenna uses a layer of metal plate and metal line as feeding structure. However, according to antenna theory, the bandwidth of the antenna is inversely proportional to the Q value, described by the following formula As a result, one of the effective way to widen the bandwidth of the microstrip antenna is to decrease the Q value of the feeding structure. Compared with common microstrip feeding structure, slot coupling, which transfers the power from feeding layer to radiation part of the antenna, could decrease the Q value and reduce the influence of patch radiation pattern. Besides, H-shaped slot could reduce the area of the slot, which is beneficial to reduce the back radiation of the patch and thus improve the cross polarization performance of the circularly polarized antenna. The view of feeding sector based on H-shaped slot is shown in figure 2. We select Teflon as substrate, and metal line and plate are on both sides of the substrate. The width and length of feeding slot (l0 and w0 shown in figure) are the most important parameters which affect the impedance matching performance. Considering the bandwidth and return loss design margin, we select 2.52mm as the width of slot after simulation. Based on the simulated results above, the feeding structure of the array is well designed. 2.2Radiation Patch DesignThere are many ways to design circularly-polarized patch, such as two orthogonal microstrip lines and feeding from bottom by coaxial line. The former way will affect the pattern of the patch due to the radiation from two lines, and the manufacturing process of the latter is complex, which is not conducive to low profile design and mass production. As a result, considering radiation performance and manufacturing process, we introduced the design of chamfered patch. In addition to broadening the bandwidth by adjusting the feed structure, parasitic patches can also be used to widen the antenna bandwidth. The lower patch acts as a radiation layer to transmit energy to the upper layer and free space, and the upper patch acts as a parasitic layer to expand the bandwidth of the microstrip antenna. Compared with single layer, double layer design could improve bandwidth performance by more than 15%, shown in figure 3. The view of radiation sector based on corner-cut patch is shown in figure 4. According to the central frequency of patch and microstrip antenna theory, the length and width of the radiaton patch are designed as following formula The length of corner could be described as In order to achieve good radiation performance and impedance matching, the size of the chamfered corner is analyzed by HFSS electromagnetic simulation software, and the final length and width of the radiation patch is 9.60mm×9.12mm. Similar to the analysis of radiation patch, the size of the parasitic patch is also analyzed and simulated by the method above, and obtain the optimal size of the parasitic patch. The size of the final double-layer patch is shown in Figure 5. 2.3Rotation ArrayThe radiation and parasitic patch are well designed, but a single double-layer patch structure could only meet the demand of the axial ratio in the central frequency and the narrow band. The axial ratio performance is poor in the wide band, with only about 10% of the axial ratio bandwidth, which cannot meet the demand of many scenarios. As a result, a 2×2 rotation sub-array is proposed. It is an effective method to realize miniaturization, wide axial ratio bandwidth and high gain. The four groups of patch and feeding lines are arranged in a clockwise rotation of 90 degrees. Simulated results verify the validity of the analysis and design above. The 2×2 rotation sub-array could achieve the range of axial ratio lower than 6dB over 100 degrees, as shown in figure 5. And the axial ratio bandwidth, which achieves the range of AR lower than 6dB over 90 degrees, is more than 1GHz, as shown in figure 6. 3.PERFORMANCE OF THE SUB-ARRAYFrom the design and analysis above, a 2×2 slot-fed circularly-polarized double-layer microstrip antenna array is proposed. The design of double-layer structure and corner-cut patch has been applied to improve circularly-polarized performance. Impedance matching is well designed by H-shaped slot and microstrip line. The simulated bandwidth for VSWR lower than 2.0 of the sub-array is 37.0% in the frequency range of 2.4GHz-3.5GHz. 10.3dBi gain is obtained at the design frequency, and the cross polarization level reaches over 40dB at the central frequency in E-plane and H-plane. The range of axial ratio lower than 6dB of the sub-array is over 100 degrees at the design frequency, and the axial ratio bandwidth (lower than 6dB) is over 1GHz. The size of the sub-array is 103.2mm×103.2mm×15.43mm. It can be the basic antenna sub-array unit to achieve massive microstrip antenna with the design of feeding network. 4.CONCLUSIONA 2×2 slot-fed circularly-polarized double-layer microstrip antenna array is proposed in this paper. Double-layer structure and corner-cut patch are designed to improve circularly-polarized performance. Impedance matching is well designed by H-shaped slot and microstrip line. The rotation array improves the radiation characteristic and axial ratio bandwidth. The simulated bandwidth for VSWR lower than 2.0 of the sub-array is 37.0%, and 10.3dBi gain is obtained at the design frequency. The axial ratio bandwidth (lower than 6dB) is over 1GHz, much wider than conventional circularly-polarized microstrip antenna. The sub-array obtained could be used in the design of wideband massive circularly-polarized antenna array. REFERENCESF. Ferrero, C. Luxey, G. Jacquemod, and R. Staraj,
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