Reduction of Tropospheric Noise Fluctuations at Centimetre Wavelengths

Abstract

SOME time ago, Conway¹ suggested the use of a special technique for radio astronomical observations at centimetre wavelengths. It should eliminate the influence of radiation fluctuations from the troposphere on the sensitivity of the radio telescope. This communication reports the success of tests at wavelengths of 9.5 mm and 2 and 6 cm on a 140 ft. telescope. The radiometer is switched between two feed horns, both located close to the focal point of the paraboloid, and the difference in power is recorded. Two similar beams are projected on the sky separated by some angle. Close to the antenna (in the Fresnel region) the cylindrical beams overlap partially. By keeping the source in one of the beams only, we can measure its flux density and simultaneously cancel the atmospheric noise, because it enters the two overlapping beams. The method is restricted to observations of sources with angular extent less than the beam separation.