Recent research activities on functional properties and applications of Ag(Nb,Ta)O3-based materials are reviewed and critically discussed. Fundamental aspects of these materials, such as crystal chemistry, lattice dynamics and phase formation, are explained first. Further, a focus is placed on the development of state-of-the-art applications for passive components, piezoelectrics and tuneable thin films. Many areas are identified where further research should focus. This includes open questions from the fundamental segment of research as well as challenges of the applied development. The review clearly presents potentials of the Ag(Nb,Ta)O3-based materials but also reveals an incomplete understanding of these materials.

Review of Ag(Nb, Ta)O3 as a functional material

In this paper, a 2-D beamforming phased array using a novel 2-D Nolen matrix network is presented. The Nolen matrix is a novel antenna feeding network composed of only couplers with dedicated coupling ratios and phase shifters. It does not require crossover and load termination compared to other networks based on Butler and Blass matrix. To be specific, the closed-form equations are derived first for uniplanar single $3 \times 3$ Nolen matrix, which is composed of three couplers and three phase delay lines. Most importantly, it is found that the proposed Nolen matrix can employ couplers with arbitrary phase differences to achieve relatively flexible progressive phase delays across the radiating elements, presenting a high degree of freedom on circuit topology and beamforming performance. Then, a 2-D antenna feeding network is designed by stacking and cascading six $3 \times 3$ Nolen matrices, and a 2-D patch antenna array is integrated with the proposed feeding network to generate nine radiation beams with unique directions on azimuth and elevation planes, realizing the 2-D beamforming function. To verify the proposed design concept, a prototype of 2-D beamforming phased array operating at 5.8 GHz is designed, fabricated, and measured, and the experimental results agree well with simulation and theoretical analysis.

A Novel 2-D $3\times3$ Nolen Matrix for 2-D Beamforming Applications

To support the ever-increasing demand on connectivity and datarates, multiple beam antennas are identified as a critical technology for the fifth generation (5G), the sixth generation (6G) and more generally beyond 5G (B5G) wireless communication links in both terrestrial networks (TNs) and non-terrestrial networks (NTNs). To reduce the cost and power consumption, there is a marked industrial interest in adopting analogue multiple beam antenna array technology. A key sub-system in many of such antenna arrays is the circuit type multiple beamforming network (BFN). This has led to a significantly renewed interest in and new technological developments of Butler matrices, Blass matrices, and Nolen matrices as well as hybrid structures, mostly at millimeter-wave frequencies. To the best of the authors’ knowledge, no comprehensive analysis and comparison of circuit type multiple BFNs have been properly reported with focus on 5 G and 6 G applications to date. In this paper, the principle of operation, design, and implementation of different circuit type multiple BFNs are discussed and compared. The suitability of these sub-systems for 5 G and B5G antenna arrays is reviewed. Major technology and research challenges are highlighted. It is expected that this review paper will facilitate further innovation and developments in this important field.

Circuit Type Multiple Beamforming Networks for Antenna Arrays in 5G and 6G Terrestrial and Non-Terrestrial Networks

This paper proposes a low-loss Butler matrix using patch element and honeycomb concept on substrate integrated suspended line (SISL) platform. The SISL Butler matrix is composed of four couplers, two crossovers, and four phase shifters. Both the coupler and crossover are implemented using patch elements for ease of fabrication and low conductor loss. We adopt the honeycomb concept to integrate all the required components together to form the Butler matrix as an integrated whole. Every component is assigned with an individual cavity, and via holes are designed between the adjacent cavities to form the via fence, which makes these components isolated from each other. All the components are embedded inside the air cavity of the self-packaged SISL multilayer structure with electromagnetic shielding property, and the radiation loss can be reduced to minimum. Double-metal layers with via-hole connection are adopted to reduce the conductor loss. The dielectric loss is further reduced by using patterned substrate. The proposed SISL Butler matrix is fabricated using the standard printed circuit board process with low cost. From 24.5 to 26.5 GHz, the measured insertion loss of the Butler matrix is around 1.1 dB, and the measured phase error is within 8°. An SISL antenna array is integrated with the Butler matrix for verification of its switch-beam property. The proposed SISL Butler matrix has advantages such as low loss, low cost, high integration, and self-packaging.

A Low-Loss Butler Matrix Using Patch Element and Honeycomb Concept on SISL Platform

In this paper, an ultra-compact low-pass filter (LPF) with wide stopband is proposed based on lumped L-C structure in through-silicon via-based 3-D stacked configuration. The spiral inductor (L), interdigital capacitor (C), and the proposed LPF are theoretically analyzed based on parasitic parameters and equivalent circuit models. To evaluate the characteristics, the proposed LPF is simulated by HFSS, fabricated with CMOS-process-compatible technology, and measured with de-embedding method. Simulation and measured results are in good agreement. Compared with the other literature, the proposed LPF has the most compact size of $0.028\lambda _{g }\times 0.017 \lambda _{g}$ . ( $\lambda _{g}$ is the guide wavelength at $f_{c}$ .) With the cutoff frequency at 10.05 GHz, the filter has an insertion loss of 0.14 dB and reflection loss over 13 dB from dc to 9 GHz in the passband. The suppression levels are better than 20 dB from 20.4 up to 40 GHz in the stopband.

Ultra-Compact TSV-Based L-C Low-Pass Filter With Stopband Up to 40 GHz for Microwave Application

A novel transition from substrate integrated suspended line (SISL) to conductor backed coplanar waveguide (CBCPW) is proposed in this letter. The proposed transition utilizing the standard PCB techniques eliminates the drawbacks of the traditional suspended circuits and integrates the suspended circuits effectively. The new transition is a self-packaged system verified by both simulation and experiment results. Back-to-back transition structure has insertion loss of less than 0.6 dB and return loss of better than 15 dB from dc to 8 GHz.

A Novel Transition From Substrate Integrated Suspended Line to Conductor Backed CPW

With the rapid development of modern communication and radar technology, there is a need for the miniaturization, planarization, and modularization of microwave and millimeter-wave (mm-wave) systems, without losing excellent performance and reliability. The transmission line is the most basic component of microwave and mm-wave circuits and systems, and its characteristics-including loss, size, integration, cost, and weight-directly or indirectly affect the performance of the entire circuit and system. Commonly used transmission lines [1] include nonplanar transmission lines, such as a metal waveguide and coaxial line, and planar transmission lines, such as a microstrip, strip line, coplanar waveguide, and slot line.

Recent Progress in SISL Circuits and Systems: Review of Passive and Active Circuits Demonstrating SISL's Low Loss and Self-Packaging and Showcasing the Merits of Metallic, Shielded, Suspended Lines

A novel compact branch-line coupler using substrate integrated suspended line (SISL) technology is proposed in this letter. In order to reduce the size of conventional branch-line coupler, we propose a novel slow wave structure, which is composed of high-low impedance lines cascaded in meander line configuration, and a 62% size reduction has been achieved. Moreover, fabricated by using standard printed circuit board process, the proposed compact SISL branch-line coupler has advantages of self-packaging, low cost and light weight.

A Compact Branch-Line Coupler Using Substrate Integrated Suspended Line Technology

This paper presents a $1\times4$ cavity-backed endfire dipole antenna array based on substrate-integrated suspended line (SISL) platform. A double-side SISL (DSISL) self-packaged feeding network with irregular boundaries is designed for the array. By utilizing the DSISL feeding network, the radiation loss and substrate loss of the feed line are effectively decreased, thus efficiency of better than 91% is achieved. Instead of the conventional metallic cavity, the proposed cavity structure is realized by multiple substrate layers. A predominantly TE10 aperture field is obtained by adjusting the sizes of the backing cavity. Measured results show that the proposed dipole antenna array has narrow beam widths in the E-plane and obtained 10 dB impedance bandwidth of 19.7%, i.e., from 22.77 to 27.75 GHz which can cover the 24 GHz automotive radar band (24.25–26.65 GHz). Moreover, front-to-back ratio of better than 26 dB, high gain of better than 14.1 dBi, and low sidelobe level of better than −15 dB are achieved over the automotive radar frequency band.

Yongqiang Wang

Papers

A Low-Loss Butler Matrix Using Patch Element and Honeycomb Concept on SISL Platform

A Novel Transition From Substrate Integrated Suspended Line to Conductor Backed CPW

Recent Progress in SISL Circuits and Systems: Review of Passive and Active Circuits Demonstrating SISL's Low Loss and Self-Packaging and Showcasing the Merits of Metallic, Shielded, Suspended Lines

A Compact Branch-Line Coupler Using Substrate Integrated Suspended Line Technology

A Cavity-Backed Endfire Dipole Antenna Array Using Substrate-Integrated Suspended Line Technology for 24 GHz Band Applications