Design, development, and initial measurements of a 1.4-mm radiometric system

The development of a 1.4-mm heterodyne radiometric system and the initial measurements taken with it are described. The
\Delta T
of the system at the antenna terminals was approximately
18.5\deg
K for a 0.25-Hz post-correlation noise bandwidth. Measurements show that the attenuation through the atmosphere at this wavelength is primarily due to water vapor, and an estimate of the zenith attenuation is given by
A (dB)= 2.8w
, where
w
is the precipitable water in centimeters. Measurements of the antenna half-power beamwidths using the sun as a source show that at 1.4 mm the atmospheric turbulence effects are not appreciable for observations through the atmosphere at zenith angles less than
45\deg
with a 15-foot antenna.