A Wideband Millimeter-wave Multibeam Array Using Compact Non-coplanar Substrate Integrated Coaxial Line (SICL) Butler Matrix

In this paper, a novel wideband multibeam array fed by a non-coplanar substrate integrated coaxial line (SICL) Butler matrix is presented for millimeter-wave (mmW) applications. The presented Butler matrix consists of SICL non-coplanar couplers, crossovers, and phase shifters, whose inner signal strips are printed on adjacent layers. Considering the weak coupling between circuits on adjacent layers at mmW bands, wideband tandem-type SICL stub-loaded couplers and crossovers, which consist of coupled SICLs on adjacent layers, are introduced here to solve this issue. Compared with SICL coplanar Butler matrix printed on the same layer, offset coupled SICLs printed on adjacent layers are utilized in the presented Butler matrix to realize a much wider bandwidth while keeping a compact size (1.5λ0 × 2.8λ0). Wideband, compact SICL-fed tapered slot antennas with opposite blind via orientations are utilized to connect the inner signal strips of SICLs printed on different layers. As demonstration, a multibeam array prototype fed by a 4 × 4 SICL non-coplanar Butler matrix is fabricated and measured, showing a desirable agreement between simulated and measured results. The presented multibeam array owns a wide operating bandwidth of 51.4% (26-44 GHz) with desirable beam steering performance. For the sake of wideband, compact, ease of integration, the presented multibeam antenna array represents a promising candidate for mmW wireless applications.