A compact design of a wideband millimeter-wave Butler matrix using integrated passive device technology

This study presents a compact, wideband, 4 x 4 Butler matrix using a two-metal-layer thin-film process for millimeter (mm)-wave 5G radio applications. Size reduction techniques are thoroughly discussed for size and performance optimization. A size-reduced quadrature coupler is designed with coplanar waveguide based length-reduced branch lines using capacitive loading effect from the ground bridges. A compact crossover is designed for a return loss of 20 dB or more with negligible crosstalk over a wide frequency range. The circuit components are arranged in an optimized space, allowing negligible coupling between nearby elements. The Butler matrix is fabricated on a quartz substrate at the center frequency of 28 GHz. The fractional bandwidth is 15.4% at a 15-dB return loss and insertion loss is measured in the range of 7.8-8.7 dB at the center frequency. The size of the core circuit is 3.375 x 2.100 m m 2 ${\rm{m}}{{\rm{m}}}<^>{2}$, which corresponds to 0.469 x 0.292 lambda g 2 ${\lambda }_{g}<^>{2}$.