Novel Dual-Band Quasi-0-dB Coupled-Line Coupler Using the Composite Right/Left-Handed Transmission Lines

A novel edge-coupled symmetric coupled-line coupler featuring the quasi-0-dB couplings at adjacently dual frequencies is presented based on the composite right/left-handed (CRLH) transmission lines. The fulfillment of dual-frequency quasi-0-dB couplings is derived from the complete backward and complete forward coupling phenomena at the higher and lower operating frequencies, respectively. To this end, the dispersion relations of the even and odd-mode CRLH lines have to be engineered in a particular manner so that: 1) the substantial attenuation level for each mode; 2) the 180° phase difference between the even and odd-mode reflection coefficients in the overlapped stopband region are obtained to accomplish the complete backward coupling; 3) the enabling curve intersection for the even and odd-mode Bloch impedances; and 4) the considerable difference between the even and odd-mode propagation constants in the left-handed regions are obtained to suppress the impedance coupling and attain strong forward coupling. Based on a thorough analysis on the even and odd-mode dispersion characteristics and Bloch impedances of the coupled lines, the operating principle and design procedure for the proposed coupler with dual-band complete couplings can be fully explained and greatly simplified. A symmetric CRLH coupled-line coupler consisting of seven coupled unit cells was experimentally developed and a shielding (metal) box was used to prevent radiation loss in the forward coupling band. Experimental results show that the maximum coupling levels are -1.3 and -0.9 dB, corresponding to the 3-dB fractional bandwidths of 15.8% and 10.8% at the lower and higher operating frequencies, respectively. Good agreement is obtained between the simulated and experimental results.