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80 GHz Radiometer Calibration System

A calibration system for an 80 GHz radiometer has been designed and constructed. During observations, calibration is obtained from a noise valve, the noise of which is fed to the radiometer via a beam switch. The calibration signal is controlled by a mechanical waveguide switch. It has been confirmed by experiment that, at millimetre wavelengths, noise valves need careful calibration. For this purpose, one can use the temperature difference of boiling argon and nitrogen. Alternatively, one can use a calibrated thermometer to measure the temperatures of two thermal radiators. In both cases the absolute calibrator is placed before the feed horn. The reference beam observes radiation coming from a piece of microwave absorber at ambient temperature. The positioning of this piece is crucial.