Compact Broadband Substrate-Integrated Coaxial Line 2-D Beamforming Network and Its Multibeam Array Antenna Applications

This article presents a compact broadband 16-way two-dimensional (2-D) Butler matrix (BM)-based beamforming network (BFN) using multilayer substrate-integrated coaxial line (SICL). In order to achieve the goal of miniaturization and wide bandwidth, an SICL vertical-plane $4\times4$ BM is implemented for the first time, and then is combined with the SICL horizontal-plane $4\times4$ BM forming the compact 2-D BM. A multilayer interconnection structure is proposed and used in constructing the vertical-plane BM to replace the crossover, and a miniaturized 3-dB coupler with staggered rows of input and output ports is also designed for the vertical-plane BM. In contrast to other previously reported 16-way 2-D BFN, the proposed one exhibits a wide operating band of 28.57% (24–32 GHz) and a compact footprint of only $6.3\lambda _{0} \times 3.4\lambda _{0}$ . In addition, the proposed SICL 2-D BFN is also convenient to be integrated with the wideband radiation elements to construct a 2-D multibeam array antenna. Prototypes of both the sub-BFNs and the 2-D multibeam antenna are fabricated and measured, the measured average insertion loss of the 2-D BFN is about 2.5 dB, which is close to that of the BFN based on a hollow waveguide, while the size and bandwidth of the proposed 2-D BFN have significant advantages compared to the hollow waveguide ones.