High miniaturization potential of slow-wave transmission lines based on simultaneous inductor and capacitor loading

This paper presents an approach to the implementation of slow-wave transmission lines with high potential for miniaturization. The unit cell of the structure is a symmetric network consisting of a (host) transmission line section loaded with a shunt capacitor at the central plane and two series inductors at the extremes. Due to the presence of two reactive elements in the unit cell (inductor and capacitor), it is possible to implement slow-wave structures with very small phase velocity (or wavelength), as compared to those achievable in single-element (inductor or capacitor) reactively-loaded lines. Therefore, a high level of miniaturization in microwave circuits implemented with these artificial lines is possible. Moreover, the combined presence of inductors and capacitors provides further design flexibility for these lines. Such enhanced flexibility alleviates the limitations of previous implementations, mainly related to the minimum or maximum dimensions of the involved elements, i.e., host line and reactive elements (typically implemented as planar semi-lumped components). To demonstrate the potential of the approach, a slow-wave microstrip line impedance inverter is designed, fabricated and characterized, and then it is applied to the implementation of a compact power splitter.