Millimetric ground-based observation of cosmic microwave background radiation anisotropy at delta = +28 degrees

Results from the third campaign of a ground-based multiband observation of the millimeter emission of the sky from Tenerife (Canary Islands) are prese nted. The instrument consists of a 0.45 m diameter off-axis telescope equip ped with a four-band multimode He-3 cooled photometer working at 1.1, 1.3, 2.1, and 3.1 mm wavelengths. The beam is well approximated by a Gaussian wi th 1.degrees 35 FWHM at all wavelengths. The wide wavelength coverage of ou r instrument allows us to characterize and reduce both the atmospheric and Galactic contamination in our data. The cosmic microwave background radiati on data is analyzed in six multipole bands whose centers span the range l = 39 to l = 134 at the two longest wavelengths (2.1 and 3.1 mm). A likelihoo d analysis indicates that we have detected fluctuations in all bands at the two wavelengths. We have evidence of a rise in the angular power spectrum from low to high. Our measured spectrum is consistent with current popular theories of large- scale structure formation, COBE, and other recent balloo n-borne experiments with similar wavelength coverage.