A high capacitive ratio RF MEMS switch, with low-actuation voltage is designed, fabricated and experimentally validated on high-resistivity silicon (HRS) substrate. Thanks to very good fabrication control of all steps and to the high dielectric constant of TiO2, a down/up capacitive ratio close to 200 is achieved with 8 V pull-in. It is also demonstrated that, using a passivated-surface HRS and semi-suspended conductors on air, the microwave losses in the CPW line are as low as 0.1 dB/mm at 20 GHz. The reported RF MEMS shunt capacitor is expected to serve as core device for phase shifting applications in the 10-20 GHz range, both for switching operations and as a variable capacitor in distributed MEMS transmission lines (DMTLs).