Guided Reflectometry Imaging Unit Using Millimeter Wave FMCW Radars

This article introduces a novel and simplified implementation of a guided terahertz reflectometry system that leverages Frequency Modulated Continuous Wave (FMCW) radar technology for imaging and sensing applications. The innovation lies in the use of a single hollow-core dielectric waveguide to directly connect the radar transceiver to the sample, eliminating the need for bulky and alignment-sensitive optical components typically required in quasi-optical setups.

The study demonstrates that by combining FMCW radar systems—known for their phase-sensitive distance measurement capabilities—with dielectric waveguides, it becomes possible to differentiate between parasitic reflections along the waveguide and true sensing signals at the probing end. This significantly improves the signal-to-noise ratio (SNR) and simplifies the system architecture, paving the way for compact, portable, and cost-effective terahertz sensing solutions.

Two radar architectures are tested:

1. A high-performance III-V semiconductor-based 100 GHz SynView radar unit, which serves as the reference, and

2. A compact, low-cost 122 GHz SiGe radar chip from Silicon Radar GmbH, illustrating the versatility of the concept.

The study incorporates 3D full-wave electromagnetic simulations to evaluate key aspects such as:

• Power coupling efficiency between the radar and waveguide

• Beam propagation and mode profiles within the waveguide

• Influence of waveguide dimensions on performance

• Imaging resolution and artefacts

The use of a thin-walled polypropylene hollow-core waveguide is central to this approach. Its design is based on anti-resonant reflection guiding, enabling low-loss propagation in the air core. Coupling efficiencies are carefully examined through simulations and experiments, revealing an efficiency of ~70% with the horn-based SynView setup and ~18% with the compact patch-antenna chip.

Imaging capabilities of both configurations are validated through raster scans of standard test targets. Results show that the simpler low-cost system can achieve resolutions close to that of the more advanced setup, albeit with some artefacts. To improve resolution, a solid immersion lens is introduced at the waveguide’s output, enabling better beam focusing and minimizing ghost artefacts. With the lens, the resolution improves significantly—from ~4.5 mm to 2 mm for the SynView setup and down to 1.4 mm for the Silicon Radar system.

Key performance metrics evaluated include:

• Maximum achievable dynamic range (~27 dB)

• Beam profile uniformity and imaging artefacts

• Working distance and lateral resolution trade-offs

Ultimately, the work proves the feasibility of a low-cost, guided terahertz FMCW reflectometry unit. It highlights its potential for scalable applications in non-destructive testing (NDT), remote sensing, and imaging across a wide range of industries. The modularity of the system—allowing different radar front-ends and waveguide configurations—adds flexibility and broadens its applicability.