چكيده به لاتين
In the present study, to build a conformal anti-jamming GNSS antenna array, at first a microstrip patch antenna should be designed. The antenna design requirements contain operation in GPS L1 and GLONASS G1 frequency bands, having circular polarization and also, small dimensions. In the antenna design process, due to the interdependence of the parameters, improving one parameter affects the desirability of others. Therefore, three microstrip patch antennas with different unique geometric features and also unique performance parameters have been designed for special applications.
The first designed antenna is a miniaturized, low-cost, multiple-feed, circularly polarized microstrip patch antenna. The proposed antenna receives both L1 and G1 bands. The use of semi-fractal geometric structure has led to its miniaturization, with a compression ratio of 37%. The final geometry of the antenna has dimensions of less than 〖1800 mm〗^2, which is counted as compact and thin antennas. In order to generate pure circular polarization, a three-feed configuration with 120° phase shift between the feeding ports have been used. Simulation of the antenna performance has been done using CST software and for validation, simulation was repeated in FEKO and HFSS software. The designed antenna has FBR of 40 dB, RHCP gain of 3.45 dB, axial ratio beam-width of 108° and phase center variation (PCV) less than 0.16 mm. In the following, based on the designed antenna, a planar 7-element antenna array has been introduced. The effects of mutual coupling, number of interference sources, polarization, the ratio of interference to signal power, and also discrete phase shift, have been evaluated. To remove the interference, a new blind adaptive algorithm with discrete periodic variable (DPVA) has been applied to the array. The proposed algorithm minimizes the output power to create null in the direction of the interference. The discrete phase shift leads to use of hybrid phase shifters, which reduces manufacturing costs. The simulation results indicate the fast and reliable convergence of the DPVA algorithm. This algorithm converges in less than 1 millisecond and with 400 iterations in the examined sample. The null depth is 90 dB, which indicates the complete elimination of the interference.
In the other part of this investigation, the theory of multiple-feed configuration has been analyzed to generate circular polarization. Then, a parametric study has been done to evaluate the effect of different parameters on the circular polarization purity. By using the results of multiple-feed configuration analysis, a four-feed, dual-band (L1, L2) microstrip antenna has been designed. This antenna has an area of 1548 〖mm〗^2 and is low-cost, compact with easy fabrication feature. By using a coupler in the feeding network, the necessary phase difference has been created in both frequencies. ADS and CST software have been used to evaluate the feeding network requirements and the antenna performance, respectively. This antenna has a bandwidth of 48 MHz, FBR of 14 dB and axial ratio beam width of more than 66° in both frequencies.
The third designed antenna is a low-cost, flexible, simple microstrip antenna with very high radiation efficiency. The introduced antenna receives L1, G1, E1 and B1 bands. The flexibility of this antenna leads to its use in a wide range of applications with different geometries. The antenna has single-feed configuration. The use of a single-feed configuration with a coaxial cable and a very thin FR4 substrate leads to easy and low-cost fabrication. A parallelogram has been created on the patch as a defect. Therefore, orthogonal modes have been generated. By conducting modal analysis, the feeding location has been determined to excite orthogonal modes and produce circular polarization. The performance of the fabricated antenna was tested, which validates the simulation results. The RHCP gain of this antenna is 5.84 dB, the FBR is 21.3 dB, the axial ratio beam width is 154°, the PCV is less than 1.2 mm, and efficiency is over 90%.
In the following, the planar and conformal 7-element array has been designed, using the third antenna. In examining the conformal effect, results show that the RHCP gain of the conformal array (8.91 dB) is higher than the planar one (7.7 dB). Also, the RHCP beam-width in the conformal array is about 15° more than the planar type. In terms of losses and efficiency, the conformal array has higher efficiency and lower losses. Finally, to further remove the interference at unwanted frequencies and angles, a flexible and low-cost FSS structure has been designed and fabricated as a frequency-spatial filter. The performance of the simulated FSS structure is in good agreement with the laboratory test results. In the spatial filter, 90% of the waves are passed in the 80° beam-width domain. In the frequency filter, this structure has a bandwidth of 180 MHz with a central frequency of 1.6 GHz.
