| With the development of the miniaturization of wireless communication equipment,the study on the miniaturization of antennas is extremely deeply studied by relevant scholars at home and abroad.There are more and more kinds of communication equipment now.In order to detect the EMI(electromagnetic interference)to various equipment of railway communication and navigation,the antenna of EMI installed on the roof of the train needs to cover multiple communication frequencies,which means that the antenna required to be wideband and miniaturized.Based on the existing techniques of miniaturization and broadband,a miniaturized and wideband antenna who looks like a bulb is designed in this thesis.This antenna is composed of two orthogonal metal sheets whose sizes are much shorter than the wavelengths of the antenna,and the sheets have been optimized.The antenna is excited by a coaxial line at the bottom of the perpendicular line.First of all,we introduce the basic principle of the antenna in this thesis,list the parameters of the antenna,illustrate the effect on parameters due to the characteristics of electrical small antennas,and list some common techniques.We put forward the original antenna based on the study of electrical small antennas and the technologies of miniaturization and broadband.After analyzing all parameters of the original antenna by simulation software HFSS(high-frequency structure simulator),we determine the values of all parameter of the antenna,analyze the main characteristics of voltage standing wave ratio VSWR(voltage standing wave ratio),gain and directivity diagram in the simulation results.The performance of the original antenna is not very ideal,when the VSWR is less than 2 the bandwidth of the antenna is 0.6?1.2GHz.In order to extend the bandwidth of the antenna,we do some work to optimize the original antenna to improve its performance.As we have found that the shapes of the sheets have great impact on the performance of the antenna after analyzing the parameters,therefore,we focus on the shapes of sheets in the optimizing by genetic algorithm.The VSWR of the antenna after optimized is less than 2.2 in the bandwidth of 0.4-3GHz,and the VSWR is less than 2 in the bandwidth of 0.7-3GHz.The gain fluctuates within the range of-1.5?1.5dB.Compared with the initial antenna,the bandwidth of low-frequency of the antenna optimized is extended.The antenna with the shape of cylinder used for EMI testing is large,its height is 25 cm,and the diameter of the cylinder is 40 cm.In this thesis,the height of the antenna which is optimized is 18.75 cm,and its width is 17.8 cm,the size is smaller than the before one.After optimizing the antenna simulation model,we made the corresponding actual antenna according to the simulation results.In order to verify the practicability of the antenna,we tested its VSWR,gain and directivity pattern at different frequencies,compared the test results with the simulation's results and the antenna existing in our laboratory.Due to manufacturing process of the actual antenna's accuracy is low and the instability of the software of HFSS.There were some differences between the results of the actual antenna and the results of the simulation,but it was still in normal range.Among the bandwidth of 0.65-2.9GHz of the actual antenna,in the case of the VSWR was less than 2,the gain of the antenna was between-1.1?1.6dB.The size of the antenna optimized was smaller than the antenna existing in the laboratory,the performance was better than the one for EMI test,which met the requirements of the thesis. |
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