Characteristics of free-space propagation near the 183-GHz H2O line

Data are presented for some brief propagation measurements in the free atmosphere at 183.3 GHz (quite near the absorption line frequency) and 171 GHz (a number of linewidths from the line frequency). The observed absorption dependency upon water vapor content is in close agreement with previously reported values for frequencies near the line center, but the attenuation reported for 171 GHz is appreciably less than that deduced from one previous free-atmosphere experiment. The attenuation was found to be distinguishably greater under cloudy conditions than when the sky was clear, but the amount of data is limited.