A novel filtering T-shaped transmission line structure and its application in the design of reduced-size Butler matrix

This work presents a novel T-shaped transmission line structure designed for physical size reduction. It is analytically demonstrated that, for a specific operation frequency, the proposed structure behaves exactly like a λ/4 transmission line. Furthermore, the open-ended part of the structure can be tuned for controlled frequency rejection, thus exhibiting a built-in low-pass filtering behavior. The applicability of the network is illustrated through a practical design of the miniaturized 4 × 4 Butler Matrix (BM). As a result, the obtained BM is reduced in size by 65% compared to the classic implementation and is also capable of low-pass filtering. The theoretical and simulated return loss, insertion loss, and phase difference are in close agreement with the corresponding measured values, which proves the effectiveness of the proposed design. The designed BM has been further investigated for practical applicability as a beamforming network using patch array antennas in a simulation environment. The designed antenna array system achieves a maximum gain of 10 dBi in boresight with a possible output steering angle of ±40°.