June 1987 | NTIA Contractor Report USARO 107-86

Millimeter-Wave Studies of Moist Air

Hans J. Liebe

Abstract: Accurate laboratory measurement of attenuation rates αx over a range from 0.1 to 10 dB/km at 138 GHz for water vapor (H2O) and its mixtures with air, nitrogen (N2), oxygen (O2), and Argon (Ar) have been performed over a temperatures range from 8 to 43°C, relative humidities up to 95% RH, and total pressures reaching 1.5 atm. A computer-controlled resonance spectrometer was employed. The results are interpreted in terms of underlying absorption mechanisms. Broadening efficiencies m for mixtures H2O + N2, H2O + O2, + Ar agree among themselves with those measured within cores of the 22 and 183 GHz H2O absorption lines. The m-factors are applied to predict what share αl of the total αx results from the complete pressure-broadened H2O spectrum. The results indicate that a substantial amount of the self-broadening term proportional to the square of vapor pressure is left unaccounted. The negative temperature coefficient of the excess absorption is consistent with a dimer (H2O)2 model. A spectroscopic data base limied to 30 local H2O lines (centered below 1 THz) contributes about 1/3 to αl at 138 GHz. An empirical formulation of the experimental findings is incorporated into the parametric propagation model MPM that utilizes a local (30x H2O, 48 x x [sic] O2) line base to address frequencies up to 1000 GHz. Predictions of moist air attenuation and delay rates by means of the revised MPM program generally compare favorably with reported (10 – 430 GHz) data from both field and laboratory experiments, except for subfreezing transmission data in the 190 to 260 GHz range.

Disclaimer: Certain commercial equipment, components, and software may be identified in this report to specify adequately the technical aspects of the reported results. In no case does such identification imply recommendation or endorsement by the National Telecommunications and Information Administration, nor does it imply that the equipment or software identified is necessarily the best available for the particular application or uses.

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