| With the development of ultra-wideband technology, research on UWB antennas gains more and more attention. In point-to-multipoint UWB communication systems, omni-directional coverage of horizontal signals is required, so the horizontal radiation pattern of the radiating or receiving antenna should be stable and omni-directional in the ultrawide frequency range. Common UWB omni-directional antennas are monopole antennas which have radiators of gradually variational structure.Design in this dissertation is based on UWB circular disk monopole antenna. Omni-directional radiation characteristic of H-plane radiation patterns should be improved as well as the maintenance of ultrawide bandwidth. Combination of theoretical analysis, computer simulation and prototype manufacture helps realize the systemic analysis of antenna design.Simulation of circular disk monopole antenna shows it has ultrawide impedance bandwidth, H-plane radiation patterns change from omni-direction to bi-direction with the increase of frequency, and cutting and turnover are two efficient miniaturization methods. The geometrical symmetric structure is the essential condition of omni-directional radiation, so antenna with multi disks crossed is proposed, and the effect of disk number on the maximal difference between gain in different direction is analyzed. The simulated results show that with the increase of disk number, the omni-directional characteristic of H-plane radiation pattern is improved, and the condition of omni-directional radiation can be met when the disk number is no less than 4. However, with the increase of disk number, the real part of input impedance gets smaller and the imaginary part gets larger which means the antenna can't maintain impedance matching in the ultrawide bandwidth.An antenna with combination of two circular disks is proposed to eliminate the effect of multi-disk structure on the input impedance with good results shown. A hollow cone is used to replace the large ground plane in the monopole antenna in order to realize miniaturization in the horizontal plane. According to the analysis above, the UWB omni-directional antenna with crossed disks is presented and simulated in the frequency domain and the time domain. The results show that the antenna has excellent UWB characteristic and omni-directional radiation characteristic, and the time-domain characteristic also meets the UWB requirement. Based on this structure, three antenna prototypes of different dimensions are manufactured and measured with results consistent with simulated ones. Excellent UWB potential of this antenna is also certified which can realize a bandwith of approximately 100:1.According to the improved UWB semicircular disk antenna, methods of broadening the frequency range and miniaturization are proposed, including nested structure of multi disks, antenna with pulling poles, resistence loading and addition of top metal circular disk. The simulated results show that these methods all can improve the impedance characteristic of the antenna to a certain extent, showing great practical value. The antennas presented in this paper can be used widely in UWB communication, electromagnetic investigation and so on. |
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