Calibrating Power Meters in the 140–220-GHz Frequency Range Using an Absolute-Power Reference Calorimeter

This study aimed to calibrate power meters in the frequency range of 140-220 GHz (WR-05 band). To this end, a WR-05-waveguide calorimeter is designed as a reference standard. The calorimeter is based on the dc-substitution method in which the absorbed power is substituted for dc power as heat. A detailed thermal analysis is performed on the dc-substitution coefficient and the heat leakage of the adiabatic waveguide, which are the dominant contributors to uncertainty, and measures to decrease the uncertainty are determined. Experiments are performed to clarify the contributions of these factors to the uncertainty. A $W$ -band power meter, to which a tapered waveguide is attached to extend the applicable frequency range from the WR-10 waveguide band to the WR-05 band, is calibrated at 0.38-0.48 mW using the calorimeter. The uncertainty of the calibration is analyzed by the Monte Carlo method. As a result, the calibration factor of the power meter is determined to be 0.15-1.36 in the WR-05 band with an expanded uncertainty of 3.6%-12.7% ($k = 2$ ).