New expression for mismatch uncertainty when measuring microwave attenuation

To find the mismatch uncertainty associated with a microwave-attenuation measurement, it is essential to determine the moduli of the source-and load-reflection coefficients at the insertion point and the moduli of the S parameters of the device under test. If the phase angles of the S parameters of the device under test are also measured, the mismatch uncertainty can be calculated from a new equation that is derived in this paper. By doing so, an example (based on the RSRE X-band-modulated subcarrier attenuator calibrator) shows that a large reduction in the total uncertainty is obtained at low values of attenuation, when calibrating a precision rotary-vane attenuator.