Design of a Millimeter-Wave 4 × 4 Filtering Butler Matrix

In this paper, a millimeter-wave 4 × 4 filtering Butler matrix is proposed, which is composed of two 180° filtering couplers, two 90° couplers, and three −90° phase shifters. The 180° filtering coupler is designed by coupling-matrix (CM) method. Based on the design of 180° filtering coupler, the topology of the Butler matrix is given and the detailed design is introduced. For demonstration, a 180° filtering coupler working at 30 GHz with a bandwidth of 600 MHz is designed, fabricated, and measured. The agreement between simulation and measurement verifies the design. A design of 4 × 4 filtering Butler matrix is further introduced, which can achieve 0°, 180°, ±90° phase difference between the output ports. Phase difference is stable and insertion loss is low in the operating frequency band.