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

This study presents a compact, wideband, 4 × 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 × 2.100 mm2 ${\rm{m}}{{\rm{m}}}^{2}$, which corresponds to 0.469 × 0.292 λg2 ${\lambda }_{g}^{2}$. [ABSTRACT FROM AUTHOR]