Measurement of Atmospheric Attenuation at Millimeter Wavelengths

It is well known that certain bands in the microwave region are attenuated considerably due to absorption by water vapour and oxygen in the atmosphere. A theory of absorption for both gases was given by Van Vleck.1 Numerous measurements have been made on the gases when confined to waveguides or cavities 2 and several when unconfined in the free atmosphere. 3 Nevertheless, there is some uncertainty regarding the line-breadth constants which should be used in calculating water vapour and oxygen absorption. In particular, at atmospheric pressure there is doubt as to the amount of absorption on the skirts of the bands where the absorption is small. The present work was undertaken to test a new method of measurement and to improve the accuracy of experimental data measured in the free atmosphere. The method of measurement is one of comparison of reflections from 907 908 TIIE B E L L SYSTEM TECHNICAL J O U R N A L , J U L Y 1 9 5 6 spaced corner reflectors whose relative reflecting properties are known. The free-space attenuation is readily calculated and any measured attenuation in excess of this represents absorption by the atmospheric gases. A description of the method and the apparatus is followed by a discussion of data taken in the wavelength range 5.1 to 6.1 mm (which includes the long wavelength skirt of the oxygen absorption band centered at 5 mm). These data, when compared with the theory, 1 indicate that the line-broadening constant of oxygen at atmospheric pressure is of the order of 600 mc.