An absolute power measuring facility for the calibration of free-field power meters at near millimetre wavelengths, is described. Accuracy is two percent at the one milliwatt level, a tenfold improvement over present freefield methods. The measuring principle is based on a photo-acoustic cell and is absolutely calibrated by substitution of low frequency a.c. power. This calibration facility is available at the National Physical Laboratory, Teddington, England. G J 2, G H
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