Design of Near-Field Coupled Leaky-Wave Lens Antennas With Flat Interfaces for Material Characterization at Sub-THz Frequencies

Abstract

This work describes the design of sub-Terahertz lens antennas that are coupled in the near field. The lenses have a flat interface, making them suitable for material characterization under plane wave incidence. A waveguide-based leaky-wave antenna feed illuminates the lenses efficiently with a Gaussian pattern over a bandwidth of 140 to 220 GHz. Then, a large permittivity hyperboloid lens converts the feed pattern into a plane wave with high Gaussicity. The use of dense dielectric materials significantly reduces field spreading effects when compared to setups with free-space propagation. Furthermore, the final lens architecture presents a flat interface, enabling direct lens-to-lens coupling for 2-port measurements with only −3 dB of coupling loss. This way, a quasi-optical Thru-Reflect-Line calibration can be performed, thereby making accurate extraction of material properties via full S-parameter matrix possible. Two materials were studied with this technique in a full-wave simulation, showcasing errors below 1 percent for permittivity and 2 percent for loss tangent, using a standard plane-wave propagation model.