Miniaturized Dual-Band Directional Couplers Using Composite Right/Left-Handed Transmission Structures and Their Applications in Beam Pattern Diversity Systems

This paper demonstrates the feasibility of dual-band compact and bandwidth-enhanced directional couplers, including both 90$^{\circ}$ and 180 $^{\circ}$ hybrids, using composite right/left-handed (CRLH) transmission structures. Novel and systematic design methodology leading to size miniaturization is proposed. By considering all possible dual-band phase transitions of the constituent transmission lines of the directional couplers, this design approach ensures the compactness of the hybrids and reduces footprint size significantly compared to the previously exhibited standard transmission-line- and CRLH-based hybrids. The proposed dual-band 90$^{\circ}$ and 180$^{\circ}$ hybrids achieve 10% and 43.7% size reduction, respectively, compared to conventional microstrip couplers in the lower band of the dual-frequency operation. In addition, the fabricated 180$^{\circ}$ hybrid is shown to have enhanced bandwidth performance in both isolation and 3-dB magnitude balance. The combination of either of the proposed couplers with the proposed dual-band CRLH-based delay line enables antenna pattern diversity with distinct beam patterns in dual bands. This serves as an effective dual-band beam pattern diversity system when connected to the antenna array. Moreover, the isolation improvement by cascading the antenna array with this assembly was measured experimentally at two operational bands: 22.7 dB at 1 GHz and 10 dB at 2 GHz.