Designs of bandwidth-controllable impedance transformers using dual transmission lines

The technique of dual transmission lines is applied to replace quarterwave transformers for impedance transformation. By varying the electrical lengths of the two lines, the bandwidths of the proposed impedance transformers are reduced and become adjustable. Two designs are presented to illustrate these characteristics. The rectangular configuration minimises the coupling between its input/output ports; whereas the ring configuration allows the two ports to be arranged in any desired directions. Simulation results indicate the larger the difference between the electric lengths of the dual transmission lines, the narrower the bandwidth of the proposed impedance transformer. The validity of the computed data is verified by measuring examples prepared for both designs. The measurement data are in close agreement with the simulation results.