Compact metamaterial based 4 x 4 butler matrix with improved bandwidth for 5g applications

Abstract

This paper proposes a novel compact 4 x 4 butler matrix (BM) with improved bandwidth based on open-circuit coupled-lines and interdigital capacitor unit-cell to develop composite right/left handed (CRLH) transmission-line (TL) metamaterial structure. The BM is implemented by the combination of compact 3dB quadrature hybrid couplers, 0dB crossover and 45 degrees phase shifter on a single FR4 substrate (epsilon(r) = 4:3 and h = 1:66 mm). The simulated and measured result shows that the return loss and isolation loss are better than 14 dB at all the ports, good insertion loss of -7 +/- 2dB, which cover the frequency range of 3.2 GHz to 3.75 GHz. The phase difference of -45 degrees, 135 degrees, -135 degrees and +45 degrees are achieved with a maximum average phase tolerance of 5 degrees. The overall dimension of the BM is 70mm x 73.7mm, which shows the compactness of the proposed design that is 75% size reduction and 8.2 times improvement in the bandwidth (550MHz) as compared to conventional BM. The CST microwave studio is used to design and perform the simulations. Additionally, the simulated and measured scattering parameters and phase differences show that they are in good agreement. This compact and improved bandwidth of the proposed BM is suitable for 5G antenna array beamforming network.