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A composite right/left handed (CRLH) transmission line (TL) phase shifter, using ferroelectric (Ba/sub 0.25/Sr/sub 0.75/TiO3) varactors as tunable element, is presented for the first time. It is theoretically and experimentally demonstrated how the unique features of CRLH TLs, enables a differential phase shift with flat frequency dependence around the center frequency. The experimental prototype is a coplanar design integrated on a high resistive Si substrate. It includes four CRLH T-unit cells and has a physical length of 3850μm. The ferroelectric varactors are realized in parallel plate version. Under 15-V dc bias applied over each varactor, the differential phase shift is flat around 17GHz and has an absolute value of 50/spl deg/.