This article presents a low-insertion-loss, high-isolation, and highly integrated silicon-micromachined waveguide crossover switch for an internal calibration subsystem in spaceborne radiometers operating over seven subbands in the 163.6–182.55-GHz range, located on the lower frequency wing of the 183.31-GHz water-vapor absorption line. The device consists of four microelectromechanical switching surfaces integrated with two waveguide couplers and is implemented as a seven-chip bonded stack, providing a compact form factor of 16 × 11.4 × 2.1 mm3 with four WR-5.1 waveguide interfaces. The micromachined prototype was characterized using a custom CNC-milled waveguide module that routes the on-chip ports to standard waveguide flanges, together with a two-tier calibration employing a multiple-offset-short calibration standard fabricated in the same process run. In the crossover state, the device exhibits an insertion loss of 0.5–0.7dB, return loss better than 24dB, and isolation exceeding 29dB across 163.6–182.55GHz. In the straight state, it achieves an insertion loss of 0.5–0.75dB, return loss better than 15dB, and isolation above 33dB. Both states meet the requirements of the intended application, and the measured results show good agreement with electromagnetic simulations. In addition, the actuator enables isolation tuning—particularly in the straight state—providing a tunable attenuation capability