A Dual-Band Butler Matrix-Based Millimeter-Wave Dual-Band Multibeam Antenna Array Using a Simple Dual-Mode Transmission Line Scheme

This article reports a millimeter-wave (mmW) dual-band multibeam antenna array (MAA) based on a dual-band Butler matrix (BM) utilizing a novel dual-mode transmission line (DMTL) structure. This kind of DMTL scheme enables the realization of distinct operating modes at different frequency bands, facilitating the design of a dual-band device through an initial determination of the frequency ratio (FR) followed by topological and structural adjustments. Compared to conventional dual-band design techniques, this scheme significantly simplifies the design process while enhancing the efficiency. Based on this scheme and the proposed DMTL structure, a dual-band BM is designed and implemented with several dual-band couplers, crossovers, and shifters as a proof of concept. The structure incorporates nested sets of BMs operating at two distinct bands, providing antenna excitations with a uniform amplitude and progressive phase. Moreover, four wideband dual-polarized antennas are designed to constitute an MAA to verify its dual-band performance. A prototype operating at 24 and 30 GHz with FR =1.25 is designed, fabricated, and evaluated. The measured results show that excellent reflection coefficients (−19.2 dB) and port isolations (15.1 dB) are achieved, ensuring a stable dual-band multibeam performance. Additionally, four beam angles of [ $14^{\circ },- 45^{\circ }$ , $46^{\circ },- 12^{\circ }$ ]/[ $12^{\circ },- 35^{\circ }$ , $35^{\circ },- 11^{\circ }$ ] and cross-polarization discrimination (XPD) of 12.8 dB are realized across the required bands.