A circular fractal ultra wideband(UWB)monopole antenna with microstrip⁃feed is proposed. The antenna con⁃structed based on Descartes circle theorem is composed of circular fractal SMT and rectangular ground plane,and designed on a fiberglass substrate(FR4)whose dielectric constant is 4.4 and thickness is 1.6 mm. The measured result of the antenna shows that,when the return loss is less than -10 dB,the frequency bandwidth of the antenna is 3.02~11.3 GHz,and the impedance bandwidth is 115.6%. The radiation patterns are measured at 4.5 GHz and 10.0 GHz in working frequency band,it shows that the antenna has good omnidirectional radiation characteristic in x⁃z plane and directional characteristic in y⁃z plane. The gain′s amplitude range measured at 3.1~10.6 GHz is 4.31~6.13 dBi. This type circular fractal antenna can be widely used in commer⁃cial UWB wireless communication system and medical image filed.%提出了一种以微带线馈电方式的圆分形超宽带单极天线。天线依据笛卡尔圆定理，由圆分形结构辐射贴片与矩形接地面构成，在介电常数为4.4，厚度为1.6 mm的玻璃纤维基板（FR4）上进行设计。天线量测结果显示在返回损耗小于-10 dB时，天线的频带宽度范围为3.02~11.3 GHz，阻抗带宽为115.6%。在工作频带内实测4.5 GHz与10.0 GHz辐射方向图，在x⁃z平面具有良好的全向辐射特性，在y⁃z平面则呈现方向性。在3.1~10.6 GHz频带内量测出的增益值振幅范围落在4.31~6.13 dBi。可以广泛应用于商业超宽带无线通信系统与医学影像领域中。
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