Compact dual-mode four-port network with quadrature-coupling and direct-thru transmission in each of the individual bands

By utilizing a pair of quasi-lumped LC tanks, in this paper, a new 50-ohm synthesized microstrip line is proposed. Differing from its conventional counterparts, the new design shows open-circuit input impedance at its attenuation pole, which isolates the synthesized line from other transmission paths. A compact dual-mode four-port network, having quadrature-coupling and direct-thru transmission properties in each of the two frequency bands, is developed based on this new synthesized line. This novel four-port network behaves like a conventional 3-dB quadrature hybrid in the low frequency band, but can be equivalent to a pair of isolated transmission lines in the high band. In addition, it is 50% the size of a conventional design. The design methodology, circuit layouts, and experimental results of the synthesized line and the dual-mode four-port network are discussed carefully in this paper.