Waveguide mounted noise diodes are used for power calibration of microwave systems at frequencies below 60 GHz. They have fairly high noise temperatures but are limited in frequency coverage to a single waveguide band.
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J. W. Bowen, "A 30 to 100 GHz solid state noise source for millimetre-wave spectrometry," Proc. SPIE 1576, 16th International Conference on Infrared and Millimeter Waves, 15760Z (1 October 1991);