An innovative approach for realization of broadband 4 times 4 Butler matrices, in which broadband symmetrical multisection 3-dB/90deg directional couplers are used as basic elements, has been presented. A new technique for realization of center crossover together with 45deg phase shifters is proposed where phase correction networks, consisting of a number of coupled-line sections, are applied. The use of the proposed phase correction networks allows to achieve equal-ripple 45deg differential phase characteristics in a wide frequency range. Numerical results for the design of broadband 4 times 4 Butler matrices consisting of multisection symmetrical 3-dB/90deg directional couplers having three, five, seven, and nine coupled-line sections in conjunction with phase correction networks having an appropriate number of coupled-line sections are given in a tabular form. A method of performance improvement for broadband Butler matrices by the combinatory use of directional couplers consisting of different number of coupled-line sections is also proposed. To verify the presented analysis a broadband 4 times 4 Butler matrix consisting of six 3-dB/90deg three-section symmetrical coupled-line directional couplers and two single-section coupled-line phase correction networks has been designed, manufactured, and measured. Results of measurements of the designed Butler matrix fully confirm the validity of the proposed technique of broadband 4 times 4 Butler matrix realization.

Broadband 4 $\times$ 4 Butler Matrices as a Connection of Symmetrical Multisection Coupled-Line 3-dB Directional Couplers and Phase Correction Networks

A novel realization of a broadband $8 \times 8$ Butler matrix having phase progressions 0°, ±45°, ±90°, ±135°, and 180° has been proposed. The presented network is composed of coupled-line 3-dB/90° directional couplers and Schiffman phase shifters that ensure its broadband amplitude and phase characteristics. The proposed solution allows for a fully planar realization without any interlayer connections, which has been achieved by both rearrangement of the matrix layout and realization of the selected crossovers as tandem connections of quadrature directional couplers. The proposed concept has been verified experimentally by the design of a Butler matrix operating within 2.5–3.5 GHz and applied in a multibeam antenna array with broadside beam.

Broadband Integrated $8 \times 8$ Butler Matrix Utilizing Quadrature Couplers and Schiffman Phase Shifters for Multibeam Antennas With Broadside Beam

A lumped-element unit cell for designing compact beam-forming networks (BFNs) is proposed. It is a reciprocal four-port network, which is lossless, isolated, and matched at all ports. The unit cell is designed to behave as planar and compact directional couplers with both arbitrary output amplitude and phase distributions, as well as crossovers with arbitrary output phase distributions. For both cases, design equations are derived. The design procedure is validated numerically and experimentally through several examples. The versatility of the unit cell allows the design of BFNs using only the proposed network topology. As an example, a 4 $\,\times\,$ 4 Butler matrix is implemented. A strong size reduction is also achieved with respect to classical designs thanks to the lumped-element implementation. The prototype footprint is approximately 25 times smaller than typical transmission-line-based Butler matrices. The structure shows also a very small area compared to state-of-the-art miniaturized 4 $\,\times\,$ 4 Butler matrix designs. Measurement results confirm the behavior of the Butler matrix as a BFN. The achieved bandwidth is approximately 8%.

A Lumped-Element Unit Cell for Beam-Forming Networks and Its Application to a Miniaturized Butler Matrix

Due to the rapid development of wireless communication technologies, the number of wireless users are radically increasing. Currently, $\sim 23$ billion wireless devices are connected to the internet, and these numbers are expected to increase manifolds in the years to come. The technology growth of the fifth-generation (5G) wireless systems will be needed to meet this high demand of the network. 5G wireless systems offer data-rates of up to 10Gbps, 1-ms latency, and reduced power consumption. It is a known fact that 5G wireless systems will be exploiting beyond the presently used 3 GHz microwave and millimetre-wave (mm-wave) frequency bands. This is the primary driver in the development of the 5G wireless system. Multi-beam Phased array antenna (PAA) systems are typically used in the deployment of 5G systems for high-gain and directionality. In current 5G and future Beyond 5G (B5G) antenna array systems, beamforming networks (BFNs) such as the Butler Matrix (BM) will play a key role in achieving multi-beam characteristics. So, this paper presents an extensive review of the BM based BFNs, and discusses which type of BM will be suitable for the phased array antenna (PAA) systems in the upcoming 5G and next-generation of B5G wireless systems. Moreover, this paper also summarizes the different types of BM designs based on the number of layers. The BMs are classified into the bi-layer, tri-layer, and four-layer structures. It includes different techniques that have been used to solve the problem of crossing, narrow bandwidth, and size reduction of the BM. From the previous studies, it is found that most of the past research work was performed using the bi-layer BM system, whereas the difficult geometries like tri- and four-layer BM are avoided due to their complex fabrication process. It is also found in this paper that the metamaterial (MTM) based bi-layer BM achieves low insertion-loss and phase-error, excellent bandwidth and compact size, and good S-parameter performance, which makes them an ideal BFN candidate for the upcoming 5G and next-generation B5G systems.

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Butler Matrix Based Beamforming Networks for Phased Array Antenna Systems: A Comprehensive Review and Future Directions for 5G Applications

An innovative approach that allows for realization of broadband planar fully integrated 8 × 8 Butler matrices is presented. Coupled-line directional couplers have been utilized, which enable broad operational band. A novel arrangement of the network has been proposed that allows the creation of an entirely planar design having two metallization layers and no interconnections between these layers. Four selected crossovers have been realized as a tandem connection of two 3-dB/90° coupled-line directional couplers, which, together with reference lines having appropriate electrical lengths, perform simultaneously crossovers of signal lines, and all needed broadband constant value phase shifters. Moreover, two of the needed 3-dB/90° directional couplers have been designed as tandem connections of two 8.34-dB directional couplers, acting as 3-dB/90° directional couplers having crossed outputs. With such a modification, a fully planar design with no inter-layer connections is possible. The proposed network arrangement has been experimentally tested with the design of an 8 × 8 Butler matrix operating at the center frequency of 3 GHz. The obtained measurement results fully confirm the validity of the proposed approach.

Broadband Planar Fully Integrated 8 $\, \times \,$ 8 Butler Matrix Using Coupled-Line Directional Couplers

This paper presents a reduced size quadrature branch-line coupler that has achieved size reduction by approximately up to 80% without having to compromise performance. This design is realized on a low cost substrate (FR4, 0.50mm, ?r = 4.4), yields an excellent return loss (?49.0dB) and coupling loss (?3.30 ~ ?3.50dB) characteristics at 1.8GHz. The practical application of this novel reduced size branch-line coupler has been implemented and demonstrated in a 4×4 Butler Matrix beam-forming network operating at 1.7-2.0GHz.

T. K Gary Kwang

Papers

4×4 Butler Matrix Beam Forming Network using Novel Reduced Size Branchline Coupler