Miniaturization of Wideband 4×4 Butler Matrix with a Modified Two-Stage Schiffman Phase Shifter

This work presents a miniaturized 4×4 Butler matrix with 15% higher effective operational bandwidth compared to a traditional Butler matrix. Miniaturization is achieved by using circular geometries for the couplers and crossovers, which effectively utilize the PCB area. Additionally, the bandwidth limitation of a traditional matrix, caused by conventional phase delay lines, is addressed by designing a modified Schiffman phase shifter based on a two-stage coupled-line approach. These improvements together result in at least a 50% size reduction. A microstrip line-based matrix has been fabricated. This technique can be applied to other planar guides, as it is based on a transmission line model. Measurement results show a return loss below 10 dB, minimum isolation of 17 dB, amplitude imbalance within ±1.5 dB, and phase imbalance within ±100 over 2.25 to 2.60 GHz, making it ideal for wireless LAN applications in the ISM band at 2.45 GHz.