118 GHz single pole double throw (SPDT) switches were designed and tested for radiometers in satellite applications. Filter synthesis method was proposed for the switches analysis. Based on equivalent circuit of the discrete AlGaAs/GaAs heterojunction PIN diodes, lumped element filter model of the switches was synthesized and its equivalent distributed circuit was developed. The size of the developed 118 GHz quasi-MMIC SPDT switch circuit is 6×2.5×0.1 mm3. For the packaged SPDT switch module, insertion loss (IL) introduced by mounting deviation of the waveguide microstrip transitions and the bonded ribbons is discussed. In the frequency range 110~120 GHz, the IL of the switch modules is less than 3.0 dB and typical value is 2.6 dB, and their isolation is higher than 22 dB. Response time, switch on time, switch off time and recovery time of the switches are lower than 18 ns, 20 ns, 10 ns and 18 ns, respectively. The 118 GHz switches can be widely used in transceiver systems.