Design Considerations of Miniaturized Least Dispersive Periodic Slow-Wave Structures

Slow-wave structures using distributed periodic inductive and capacitive loadings have found many microwave circuit applications as left-handed (bandpass) or right-handed (low-pass) transmission lines. A large slow-wave factor (SWF) could result in a much smaller passive component, but also a much lower bandgap (cutoff) frequency and a larger dispersion. This paper addresses the issues and the design tradeoff between the SWF, group delay (dispersion), and the cutoff frequency of a right-handed (low-pass) quasi-lumped transmission line. A new two-layer transmission line structure using 3-D substrate metallization with an SWF of 5.8 is designed. A prototype of a 3-GHz branch-line coupler with a 70% size reduction using such a transmission line structure is fabricated and tested.