A Flux-Density Scale for Microwave Frequencies

Accurate flux-density measurements of the thermal radio source DR 21 have been made at centimeter wavelengths relative to the KPW absolute flux-density scale based on Cas A and at millimeter wavelengths relative to absolute brightness-temperature measurements of Jupiter and Saturn. The absolute spectrum of DR 21 thus defined has the form S = 26.8 - 5.6 log (GHz) for v 7 GHz, and ties together two formerly independent flux-density scales. With an accuracy of about 3 percent, this spectrum of DR 21 defines a flux-density scale that can be used to calibrate antennas having beamwidths between 1 and 6 minutes of arc at microwave frequencies above 7 GEIz where other methods of absolute calibration are much less accurate.