Design and realization of a compact substrate integrated coaxial line butler matrix for beamforming applications

This article presents the modeling and realization of a compact substrate integrated coaxial line (SICL) based butler matrix operating at 5 GHz for beam-forming applications. The proposed 4 x 4 butler matrix is developed using SICL-based hybrid coupler, crossover, and phase shifter. A compact 90 circle coupler comprising of center tapped unequal stubs is designed to enhance the size reduction as well as to extend the out of band rejection. Wideband SICL-based crossover operating from DC to 10 GHz is conceived for the proposed butler matrix using a plated through hole as transition. The SICL crossover features very high measured isolation of 65 dB owing to the reduction in coupling between the two signal paths within a lateral footprint of only 0.034 $\lambda_g<^>2$. A meandered SICL-based line is used in order to provide the necessary 45 circle and 0 circle phase shift to realize the butler matrix. The fully shielded and self-packaged compact 4 x 4 SICL-based butler matrix is fabricated and experimentally validated to operate at 5 GHz.