Compact 3-D-Printed 4 x 4 Butler Matrix Based on Low-Cost and Curing-Free Additive Manufacturing

This letter presents a $4 \times 4$ Butler matrix realized using stereolithography 3-D printing technology and liquid metal filling. The matrix is in coaxial technology and relies on circular branch lines and curved $50~\Omega $ lines. Input and output ports are aligned and placed on the same plane, while branch lines are organized using all three dimensions to minimize the length of the connecting lines. A prototype, designed at an operating frequency of 12 GHz, is manufactured and tested, showing a footprint of only $1.7\times 1.3\,\,\lambda _{0}^{2}$ . A good agreement between simulated and measured transmission coefficients is observed regardless of the selected input port. These encouraging results open the door to the realization of compact low-cost and high-performing RF beam-steering networks in coaxial technology, based on curing-free additive manufacturing processes.