A Low-Loss, 77 GHz, 8 × 8 Microstrip Butler Matrix on a High-Purity Fused-Silica (HPFS) Glass Substrate

Area of Science:

• Microwave Engineering
• Materials Science
• Radar Technology

Background:

• Automotive radar systems require high-performance beamforming components.
• Existing Butler matrices often face limitations in terms of size, loss, and substrate material suitability for high-frequency applications.
• There is a need for compact, low-loss Butler matrices operating at automotive radar frequencies (e.g., 77 GHz).

Purpose of the Study:

• To develop and characterize a novel, compact, ultra-thin, passive, 77 GHz, 8x8 microstrip Butler matrix.
• To utilize a high-purity fused-silica (HPFS) glass substrate for enhanced performance at 77 GHz.
• To demonstrate a low-cost microfabrication method for the developed Butler matrix.

Main Methods:

• Design and optimization of the 8x8 Butler matrix and its building blocks (hybrid couplers, crossovers, phase shifters) using 3D electromagnetic finite element method (FEM) simulations.
• Utilizing Advanced Design System (ADS) software for rigorous electromagnetic simulations.
• Validation of the design by fabricating and simulating a 4x4 Butler matrix and comparing results with existing literature.
• Development of a low-cost microfabrication process employing a standard lift-off technique.

Main Results:

• The developed 77 GHz, 8x8 Butler matrix is compact (19.1 mm x 26.6 mm) and ultra-thin (200 μm substrate).
• The HPFS glass substrate, with its very low loss tangent (0.0005 @77 GHz), enables excellent performance, achieving return loss and isolation below -20 dB.
• Simulations of a 4x4 version showed lower insertion loss and a smaller footprint compared to published data, validating the design approach.
• A low-cost microfabrication method using a standard lift-off process was successfully developed.

Conclusions:

• A high-performance, compact, and low-loss 77 GHz, 8x8 microstrip Butler matrix on HPFS glass has been successfully designed and simulated.
• The use of HPFS glass substrate is highly effective for achieving superior performance at 77 GHz.
• The developed microfabrication technique offers a cost-effective solution for producing these devices.
• The technology holds potential for scaled versions in future 5G beamforming applications.