Compact 4 x 4 butler matrix with non-standard phase differences for IoT applications

Butler matrices represent a popular class of feeding networks for antenna arrays. Large dimensions and the lack of flexibility in terms of achievable output phase difference make conventional Butler structures of limited use for modern communication devices. In this work, a compact planar 4 x 4 matrix with non-standard relative phase shifts of -30, 150, -120, and 60 degrees has been proposed. The structure is designed to operate at the centre frequency of 2.45 GHz. Small dimensions of 31.3 x 22.9 mm make it useful for Internet of Things applications. The structure operates from 2.35 to 2.55 GHz, which covers the industrial, scientific and medical bandwidth. At the centre frequency, the measured amplitude and phase imbalance are 1.65 dB and +/- 4.3 degrees, respectively. The proposed circuit has been compared to the state-of-the-art structures from the literature.