| Several types of Vivaldi antennas for UWB system are studied in this paper. During more than twenty years since Vivaldi antenna has been proposed, the structure of feed has been improved by Gazit, and the crosspolarization level has been reduced by Langley. These improvements made the performance of the Vivaldi antenna become to be satisfactory. The work in this paper is to further improve the performance of the antenna or to reduce the size with no degradation of its performance.The application background of UWB technique and the requirements of the UWB antenna were introduced briefly in this paper, and many kinds of Vivaldi antenna were compared. In order to simulate in HFSS, it is necessary to use VBscript to model the antenna, which was introduced in this paper. For there are kinds of methods to treat the antenna end, this paper focused on three types, the original structure, the widened end structure and the arc-like end structure. The original structure has higher gain and larger dimension. The widened end structure reduced the size, however it also sacrificed the performance of the antenna, the gain of this structure is smaller and the characteristic of the crosspolarization level is a little higher. The Vivaldi antenna with arc-like end did not show better performance than the Vivaldi antenna with widened end, the reason may be the arc selected for the end is not fit. Compared to references existed, the antenna presented in this paper is smaller, and the performance in frequency domain has been improved to some extent. Finally, for the special requirement of the UWB system, CST was used to simulate the two-port network formed by transmit antenna and receive antenna, pulse response and phase of the transmitting coefficient was also studied. According to the simulation result, we can conclude that both the parameters in frequency domain such as VSWR, radiation pattern and the parameters in time domain such as pulse response, phase center can meet the requirement of UWB system. |
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