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Solar radiation and atmospheric attenuation were measured at 4.3-mm wavelength The sun was scanned with a radio telescope consisting of a 10-foot precision paraboloid antenna (6.7-minute beamwidth) and a Dicke-type radiometer. Atmospheric attenuations were determined from the change in received solar radiation with changing elevation of the sun and from direct measurements of the thermal radiation from the atmosphere. The measured attenuations at the zenith for clear skies were between 1.6 and 2.2 db. At 4.3-mm wavelength, the sun (when it is free of sunspots) appears to be a uniform disk nearly one per cent larger than its optical size. From a large number of measurements over a period of 6 months, the solar brightness temperature was found to be 7000 Â°K with an uncertainty of about 10 per cent. Sunspot regions are slightly brighter than the quiet areas; the largest observed enhancement is 2 per cent.