Design of a Compact Wideband Butler Matrix Using Vertically Installed Planar Structure

In this paper, a novel 4×4 Butler matrix with wide bandwidth and compact size is presented. The existing coupled-line-based quadrature coupler cannot be used to construct a Butler matrix due to the location of output ports. To solve this problem, a swap is introduced in a vertically installed planar structure to implement the quadrature coupler and a crossover required by a Butler matrix. For demonstration, the proposed coupler and crossover and the corresponding Butler matrix operating at 1.8 GHz are designed, fabricated, and measured. The implemented Butler matrix exhibits a wide bandwidth of 56.4% defined by 10-dB return loss and 1-dB insertion loss variation, by occupying a circuit size of 0.25λg × 0.74λg. Simulated and experimental results are in good agreement.