A three-port planar multiple-input-multiple-output (MIMO) antenna system with pattern diversity based on the complementary theory is presented in this letter. Three directional antennas without any directors or reflectors are introduced as the radiating elements, which are rotationally symmetrical and sharing a small ground. The unilateral radiation pattern is achieved for each antenna element based on the complementary theory, where an electric dipole and a magnetic dipole are placed orthogonally and excited simultaneously. An inverted L-shaped stub is used as the electric dipole. A capacitively loaded loop operated on the zeroth-order resonant mode is applied as the equivalent magnetic dipole. The capacitive loading is realized with six periodical interdigital left-handed series capacitors. Furthermore, a defected ground structure between each two ports is adopted to further improve the isolation. The operation mechanism of the proposed MIMO antenna is discussed and experimentally verified. The proposed MIMO antenna system has a measured fractional bandwidth of 7.2% (3.33–3.58 GHz) with |S11| < −10 dB. The port isolation is better than 15 dB and the envelope correlation coefficient is less than 0.0234 in the operation band.

中文翻译：

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基于互补理论的方向图分集紧凑型平面多输入多输出天线

本文介绍了一种基于互补理论的具有方向图分集的三端口平面多输入多输出 (MIMO) 天线系统。引入了三个没有任何导向器或反射器的定向天线作为辐射元件，它们旋转对称并共享一个小接地。基于互补理论的每个天线元件实现单边辐射方向图，其中电偶极子和磁偶极子正交放置并同时激发。倒L形短截线用作电偶极子。在零阶谐振模式下运行的容性负载环路用作等效磁偶极子。容性负载是通过六个周期性的叉指左手串联电容器实现的。此外，每两个端口之间采用有缺陷的接地结构，进一步提高隔离度。讨论和实验验证了所提出的 MIMO 天线的运行机制。建议的 MIMO 天线系统具有 7.2% (3.33–3.58 GHz) 的测量分数带宽，|S11| < -10 分贝。工作频段内端口隔离度优于15dB，包络相关系数小于0.0234。