Tunable asymmetric composite right-/left-handed transmission line directional coupler controlled by applied voltage

A novel electronically-controlled asymmetric directional coupler is presented. This asymmetric structure is composed of a conventional microstrip line and a composite right-/left-handed (CRLH) transmission line incorporating varactors for voltage-controlled operation. Tunable working frequency is achieved as modulating the capacitances of the structure by adjusting the bias voltage applied to the varactors. This directional coupler demonstrates tunable broad bandwidth and tight coupling characteristic by varying the varactors bias voltages from 0V to 12V. Measured and simulated S-parameters versus bias voltage relationship are shown. The loss effect of the varactor is also discussed.