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

Abstract

A low-loss, compact, ultra-thin, passive, 77 GHz, 8 × 8 microstrip Butler matrix on a 200 μm thick high-purity fused-silica (HPFS) glass substrate embedded in 0.8 μm thick patterned gold conducting layers was developed for low power automotive radars. The first-of-its-kind, HPFS, glass-based Butler matrix comprised 12 hybrid couplers, 16 crossovers, and 8 phase shifters in a footprint area of 19.1 mm × 26.6 mm. The device and the corresponding building blocks were designed and optimized using 3D electromagnetic finite element method (FEM) simulations using the Advanced Design System (ADS) from Keysight™ Technologies. Due to the very-low-loss tangent of the HPFS glass substrate (0.0005 @77 GHz) compared to other common substrate materials and rigorous design optimization, the return loss and isolation of the input ports are both below –20 dB, respectively, as verified by 3D FEM simulations. Due to the absence of any published data on a 77 GHz 8 × 8 Butler matrix, the design was validated by developing a 4 × 4 version of the Butler matrix using the same building blocks and comparing the 3D simulation results in ADS with results published elsewhere that showed that the developed Butler matrix offers lower insertion loss in a 10% smaller footprint area. A low-cost microfabrication method has been developed to fabricate the devices using a standard lift-off process. A scaled version of the device can be used for 5G beamforming applications.

Design and Implementation of C-Band Large-Power Planar Butler Matrix in SRS

“…The beamforming networks are an essential part of an SRS transmitting antenna array. There are many beamforming networks reported [1], like the Rotman lens, Blass matrix, Nolan matrix [2], and Butler matrix [3][4][5]. Compared with these passive beamforming networks, the Butler matrix is the most popular choice, given its more straightforward and cost-effective structure and wider bandwidth [6].…”

“…Compared with these passive beamforming networks, the Butler matrix is the most popular choice, given its more straightforward and cost-effective structure and wider bandwidth [6]. In addition, the Butler matrix has the advantages of reliability and reciprocity; therefore, it is widely used in several applications, such as SRS, Internet of Things (IoT), Wi-Fi, base stations, satellite communications, and automotive radars [3].…”

“…The study of the Butler matrix [7][8][9] involves many aspects, such as the perspective of the frequency band, 77 GHz [3], Ka-band [10], K-band [11], X-band [12], C-band [13], and S-band [13]. From the perspective of large-power design [14], a specific computer-aided method for a large-power Butler matrix design is given [15].…”

“…However, the most challenging problem of the Butler matrix design is the presence of path crossings, which may lead to large dimensions when directly using SIW [7]. The fabrication of the substrate-integrated waveguides is complex and expensive [3]. In some contexts, it is preferred to microstrip technology for its higher immunity to interference, no air radiation, and lower dispersion.…”

“…The measured average and peak microwave power are 500 W and 6 kW, respectively. The whole unit was machined with a modified non-planar structure with a mechanical size of 478 × 362 × 264 mm 3 .…”