Measurements of scattering parameters of devices with rectangular waveguide ports and coaxial line ports have been investigated. Unknown "thru" calibration was employed for characterizing a transition for converting the coaxial line test port to the waveguide. Transmission phase of the unknown "thru" must be known to within ±90° in the calibration procedure. A transition from the coaxial line to the waveguide used as the unknown "thru" was designed with an electromagnetic field simulator, and simulated values of the transmission phase was adopted as the required known values. The scattering parameters of a waveguide section with a coaxial probe were measured with a fabricated unknown "thru."