Visualized Design of Antenna Decoupling Networks Constructed by Cascaded Coupled Lines

This article presents a visualized approach to efficiently designing an antenna decoupling network constructed by cascaded coupled transmission lines (CTLs). The cascaded CTL architecture is explored to decouple tightly-packed antenna pairs with extremely small feed point spacing, circumventing the narrow bandwidth and loss issues associated with resonant decoupling structures. To facilitate the fast CTL design, we propose to analytically map the CTLs' odd-mode (OM) and even-mode (EM) characteristic impedances onto the Smith charts of the antenna's OM and EM input impedances. By leveraging this approach, the required CTL's OM and EM parameters for antenna decoupling can be directly determined without relying on intricate optimization or try-and-error process. To demonstrate the effectiveness of our methodology, a printed monopole antenna pair with an element center-to-center spacing of merely 0.1.0 (at 3.5 GHz) is prototyped, which exhibits a 15-dB isolation bandwidth of 18% and a total efficiency exceeding 85% over this bandwidth. The proposed decoupling approach substantially simplifies the design of compact high-isolation antenna pairs which would have important applications in multiple-input multiple-output (MIMO) communication systems.