Primary characteristic of a capacitive MEMS switch is the ratio of capacitances in the open and closed states. Conventional switches have this ratio from several units to several tens. However, it can be significantly increased by mounting a “floating” electrode onto the transmission line. The analytical approach provides the capacitance ratio of the modified switch as high as 105. Finite element simulation takes parasitic capacitance into account and gives significantly lower value. In this work, the dependence of capacitive characteristics and S-parameters on the substrate properties is investigated. The ways for enhancing the switch performance are proposed.