Ultra slow-wave periodic transmission line using 3D substrate metallization

A novel ultra slow-wave transmission line structure using three-dimensional periodic substrate metallization is proposed, where each unit cell is made up of a multilayer loop as a series inductor and composite inter-digital strips as the shunt capacitor. Design trade-off between high slow-wave factor (SWF) and small group delay (dispersion) is addressed. A transmission line on a 4-layer FR4 substrate is shown with a slow-wave factor of 14. Using such an ultra slow-wave right-handed structure, a prototype of 2.4 GHz branch-line coupler with 86% size reduction compared to the microstrip line implementation is fabricated and tested.