Compact planar microstripline branch-line and rat-race couplers

TLDR

Branch‑line and rat‑race couplers are planar and use only transmission lines, but their 90°/270° electrical lengths require large circuit area. This work develops size‑reduced branch‑line and rat‑race couplers by replacing conventional transmission lines with artificial transmission lines. The artificial transmission lines are built entirely from microstriplines, enabling easy fabrication with standard printed‑circuit processes and are described by explicit design formulas. Full‑wave simulation and measurements at 1.8 GHz confirm the design, showing that the hybrids’ frequency response matches that of conventional couplers.

Abstract

Both branch-line and rat-race couplers are easily realized using planar circuit technology as they employ only transmission lines without additional components. However, as the electrical lengths of the transmission line elements are either 90/spl deg/ or 270/spl deg/, such couplers consume a significant amount of circuit area. This paper shows the development of branch-line and rat-race couplers that use artificial transmission lines (ATLs) in place of conventional transmission lines resulting in significant size reduction. As the ATLs are constructed entirely from microstriplines, the couplers are easily fabricated using conventional printed-circuit processes. The design formulas developed for the ATLs are explicit. Full-wave simulation and experimental results were used to confirm the design approach for hybrids operating at 1.8 GHz. The frequency response of the proposed hybrids is similar to conventional hybrids.