Ultra-fast planar GaAs Schottky diode switches provide outstanding isolation and insertion loss for G-band radar receiver protection applications. This article presents the design and measurements of a tunable SPST waveguide switch using a novel solid-state architecture with planar GaAs Schottky diodes. The switch is tunable over a 158 to 175 GHz operational bandwidth. An isolation > 43 dB at 0 dBm input power and >30 dB at +17 dBm input power was demonstrated. At 0 dBm input power, the ON-state insertion loss was under 0.86 dB and return loss exceeded 20 dB. The switch operates at ultra-high speeds, as demonstrated by a 4 MHz switching setup.