A novel design methodology for asymmetric coupled line passive devices and its application in impedance transformers

This article presents a novel design methodology for asymmetric coupled line passive devices. Based on the extraction of distributed parameters of coupled lines from S-parameters, we re-derived modal characteristic parameters and Z matrix of four-port asymmetric coupled lines in inhomogeneous mediums. Then, Z matrices of ten two-port asymmetric coupled line circuits are derived and their frequency responses are analyzed. Inspired by the image parameter method, image impedances are adopted for two-port device designs. Data diagrams which connect geometric parameters with impedances are provided to simplify design processes. Two impedance transformers utilizing the open-ended and short-ended coupled line circuits, respectively, producing high transforming ratios of 100 (50 Ω to 0.5 Ω) and 40 (50 Ω to 2000 Ω) are fabricated to verify the proposed analysis and design method. Excellent agreement between the simulation and measurement result proves the unique superiorities of asymmetric coupled lines with better initial values.