A tunable capacitor using an immiscible bifluidic dielectric

A tunable capacitor using two immiscible fluids of different dielectric constants as a dielectric material are investigated and the feasibility of its RF application is demonstrated. Large capacitance change is obtained when the two fluids have large differences of dielectric constant. When deionized water and air are used as the two fluids, the tunability (T[%]=100×(C_water-C_air)/C_air) shows a maximum of 6700% at low frequency. Silicone oil (dielectric constant of 2.5) and air is used for the high frequency application to ensure low dielectric loss in the RF frequency range. Nominal tunability and Q-factor at 2.5 GHz is 15.7% and 17.4, respectively. Barium strontium titanate (BST) nano powder mixture with silicone oil (43 wt% of BST) shows 12% more tunability than that of a pure silicone oil capacitor (due to the high permittivity contribution of BST) without concomitant Q-factor degradation. A tunable resonator consisting of the fluidic tunable capacitor combined with a line inductor is fabricated and a tuning bandwidth of 15% (0.27 GHz) is obtained at 1.8 GHz.