A Miniaturized Metamaterial-Based Dual-Band 4×4 Butler Matrix With Enhanced Frequency Ratio for Sub-6 GHz 5G Applications

Abstract

This paper introduces an innovative $4 \times 4$ dual-band Butler matrix (BM) characterized by compactness and an enhanced frequency ratio (K). The design employs meandered lines and an interdigital capacitor (IDC) unit-cell-based composite right/left-handed transmission-line (CRLH-TL) metamaterial (MTM) structure. The BM integrates compact dual-band 3 dB branch-line couplers (BLC), a 0 dB crossover, and dual-band ±45° phase shifters on a single Rogers RT5880 substrate having relative permittivity $\varepsilon _{r}$ of 2.2 and thickness $h$ of 0.787 mm. Simulations and measurement results demonstrate reflection and isolation coefficients exceeding −20 dB at all ports, with obtained insertion loss of −6±3 dB over the 0.7 GHz and 3.5 GHz frequency bands. The achieved output phase differences of ±45°, ±135°, ±135°, and ±45° at the designed frequencies indicate a maximum average phase tolerance of ±4.5° concerning the ideal values. Importantly, the BM’s overall dimensions are 143 mm $ \times 186$ mm, resulting in an impressive 78% size reduction compared to traditional T-shaped BM designs. The proposed configuration is designed and simulated using CST Microwave Studio, with the agreement between simulated and measured parameters highlighting design reliability and effectiveness. Additionally, a performance evaluation comparing the proposed BM with existing circuits reveals its suitability for sub-6 GHz 5G dual-band antenna array beamforming networks (BFN) due to its compact size and improved band ratio.

Keywords

• Branch-line coupler
• butler matrix
• composite right/left-handed
• crossover
• frequency ratio
• metamaterials