High-Quality Factor Air-Void Inverted Shielded Microstrip Transmission Lines on Low-Cost Substrate

A three-layer Air-Void Inverted Shielded (AVIS) transmission line topology is presented for low-cost printed circuit board (PCB) environments. The architecture utilizes a face-down, inverted conductive trace embedded along the lower boundary of an upper substrate layer, leaving the active signal line suspended over an evacuated longitudinal channel routed into an unclad middle spacer layer. A vertical metallic shielding array embedded within a base substrate is used to capture the electric field. To validate the framework, prototypes were fabricated on standard multi-layer FR4 substrates and characterized across the 0.5–2.5 GHz spectrum. Experimental measurements demonstrate an impedance-matched, quasi-TEM, low-dispersion effective relative permittivity trajectory (εreff≈3.1 with less than 2% variation across 0.5–2.5 GHz) paired with an unloaded quality factor (Q) scaling up to 125 at 1 GHz.