Measurement of the intensity of the cosmic background radiation at 3.0 cm

The intensity of the cosmic background radiation (CBR) was measured at a wavelength of 3.0 cm as part of a program to measure the Rayleigh-Jeans spectrum of the CBR at five wavelengths between 0.33 cm and 12 cm. The instrument used is a dual-antenna Dicke-switched radiometer with a double-sideband noise temperature of 490 K and a sensitivity of 46 mK/Hz/sup 1/2/. The entire radiometer is mounted on bearings. The atmospheric emission was measured by rotating the radiometer, and thus directing one antenna to zenith angles of +/-30 and +/-400. The CBR temperature is found by direct comparison with a liquid-helium-cooled absolute-reference load. This load consists of a large cylindrical waveguide, 70 cm in diameter and 130 cm long, terminated by a microwave absorder immersed in LHe. Only two thin polyethylene windows are between the antenna and the LHe. The load has extremely low loss at all five wavelengths of observation. At 3.0 cm reflected power and insertion loss from the walls and windows contribute an additional 20 +/- 10 mK to the antenna temperature of the LHe