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 vertically installed planar (VIP) structure to implement the quadrature coupler and crossover required by a Butler matrix. For demonstration, the proposed coupler, crossover and 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 0.25 λg ×0.74 λg. Simulated and experimented results are in good agreement.