Performance characteristics of a 300-GHz radiometer and some atmospheric attenuation measurements

A 300-GHz Dicke-type superheterodyne radiometer receiver was used for measurements of atmospheric attenuation of electromagnetic waves over an open path at frequencies near 300 GHz. The average measured values of horizontal attenuation at 304 GHz and 316 GHz, presumably due to atmospheric water vapor absorption, were, respectively, 3.35 dB/km and 5.55 dB/km per g/m
³
of water vapor density. Absorption variations at 304 GHz with respect to water vapor density were shown in the measured results. The variation of the effective zenith sky temperature with respect to atmospheric water vapor density was also determined. The minimum detectable temperature difference
(\Delta T)_{min}
, was obtained by measuring the rms value of noise in the receiver output. The best value achieved was
3.16\deg
K. Based on this result, the receiver noise figure and the mixer conversion loss were determined indirectly. The results were 31.4 dB and 22.9 dB, respectively. A blackbody radiation source served to calibrate the radiometer.