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This paper deals with the microwave study of coplanar tunable capacitors using Mn-doped Ba0.6Sr0.4TiO3 ceramics. These ceramics are developed to improve performance at microwave frequencies (i.e., compromise between tuning capabilities and insertion losses). Interdigitated capacitors were first fabricated on thick Mn-doped BST ceramics. The capacitors showed reduced tuning factor because of a parasitic capacitance between the contact pads. The use of Mn-doped BST/ SiO2 bilayers led to a significant enhancement of the capacitor performance (57% of agility under 200 V at 1 GHz).