Compact Broadband High-Directivity Microstrip Directional Coupler

Abstract The difference between phase velocities of the odd and even modes in inhomogeneous microstrip coupled lines makes the microstrip directional coupler a poor device regarding directivity. In this work, this issue is successfully compensated by the addition of a matching network at the ports of coupled lines. Furthermore, the proposed matching network, composed of a single segment of microstrip line, was extensively studied providing significant insights for the design of a compact and broadband device. As result, simple design equations based on coupled lines S-parameters are obtained, as well as the achievable operation limits of the proposed matching network. Finally, in order to validate the proposed design methodology, a compact microstrip directional coupler is constructed to operate with directivity higher than 30 dB and coupling of 17 dB over a 600-MHz bandwidth.

摘要：非均匀微带耦合线（inhomogeneous microstrip coupled lines）中奇偶模（odd and even modes）的相速度存在差异，导致微带定向耦合器（microstrip directional coupler）的方向性性能欠佳。本研究通过在耦合线端口增设匹配网络（matching network），成功弥补了这一不足。此外，所提匹配网络由单段微带线构成，本研究对其展开了全面深入的分析，为紧凑型宽带器件的设计提供了重要参考依据。最终，本研究推导得到基于耦合线散射参数（S-parameters）的简易设计公式，并明确了所提匹配网络的可行工作极限。最后，为验证所提出的设计方法，本研究制作了一款紧凑型微带定向耦合器，其在600 MHz带宽内可实现优于30 dB的方向性以及17 dB的耦合度。