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This work advances the capabilities of accurately quantifying the microwave noise temperature that exits port one of a two-port active device when port two is terminated with a complex load of known temperature. This noise temperature is of interest when characterizing and designing field-effect transistor (FET)-based cold noise sources or active cold loads that can be used as radiometer calibration reference standards. Unlike prior efforts, noise wave theory is used herein to derive an equation that embodies the complete effects of load mismatch, thus providing a new expression that correctly predicts the available noise temperature exiting port one of the two-port device. An electronic tuner is used to vary the impedance presented to port two of an on-wafer device while the port-one noise temperature is measured. The measured forward noise parameters of the device, the tuner impedance, and the tuner temperature (usually ambient) are used to compute the predicted output noise temperature values. Good agreement was observed between the noise wave-based predicted noise temperature and the measured noise temperature. Equations are developed for achieving minimum noise temperature, and a procedure including simulations and a flowchart are also presented for the design of FET-based synthetic cold loads.