Multimode CRLH SIW Transmission Lines for Wideband Multiport Directional Couplers and Miniaturized Beamforming Networks

In this article, a multimode composite right/ left-handed (CRLH) substrate-integrated waveguide (SIW) transmission line technology is proposed, which aims to broaden the operating bandwidth of the multiport directional couplers (DCs) and solve the oversize of the beamforming network (BFN). Its core is to etch multiple interdigital slots (IDSs) on the conductor surface of the multimode SIW transmission line, which can control the electromagnetic fields of different modes in the transmission line under the condition of less electromagnetic energy leakage. By adjusting the position and size of the IDSs, the desired phase constants and electromagnetic field distributions of different modes can be obtained simultaneously. Based on the proposed multimode CRLH SIW transmission line, the six-port, the eight-port, and the ten-port DCs are designed, respectively. Compared with the traditional multiport DCs based on the SIW, the bandwidth of the proposed six-port and eight-port DCs is broadened by about 87.3%. Based on the proposed multiport DCs, the miniaturized BFNs and the miniaturized multibeam array antennas (MBAs) can be realized by adding appropriate fixed phase shift. Finally, to verify the feasibility of applying multiport DCs based on the multimode CRLH SIW transmission lines to the miniaturized BFNs and the miniaturized MBAs, a 3
$\times3$
BFN, a
$4\times4$
BFN, and a three-port MBA are fabricated, measured, and discussed. Their sizes are reduced by 58%, 49%, and 60%, respectively, compared with their SIW butler matrix (BM) counterparts.