A novel design of compact broadband 4 x 4 Butler matrix-based beamforming antenna array for C-Band applications

In this paper, a compact broadband 4 x 4 Butler matrix (BM) as a feeding network for antenna array is presented and studied. The broadband performance of the BM occurs when it operates in the almost equal difference of phases and coupling throughout the operating band. The different components of the proposed design such as branch-line coupler, crossover, and Schiffman line phase shifter are designed and optimized to be low loss and wideband. The total size of the BM is 37 mm x 47 mm, equal to 0.64 lambda(0) x 0.81 lambda(0) in design frequency at 5.2 GHz. The measurement bandwidth for fabricated structure on the FR4 substrate is 24 % achieved from return loss lower than -10 dB. The beam directions are measured at -15, -43.2, +43, and +15 degrees at 5.2 GHz for each input port 1-4 excitation respectively with the gain value of 10.2 +/- 0.2 dB and sidelobe levels of -8.2 +/- 1.2 dB. The Final BM exposures phase errors of 3 +/- 1.7 degrees with the amplitude imbalance of less than +/- 1 dB over the operational band.