Hans J. Liebe; Donald H. Layton

Abstract: Laboratory measurements have been performed at 138 GHz of water vapor attenuation αx for pure vapor (H2O) and its mixtures with air, nitrogen (N2), oxygen (O2), and Argon (Ar). Temperatures ranged from 8 to 43°C, relative humidities from 0 to 95% and total pressures reached 1.5 atm. A computer-controlled resonance spectrometer was employed. The results are interpreted in terms of underlying absorption mechanisms. Broadening efficiencies m of mixtures H2O + N2, 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 α of the total αx results from the complete pressure broadened H2O spectrum. 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. An empirical formulation of the experimental findings is incorporated into the parametric propagation model MPM that utilizes a local (30x H2O, 48x O2) line base to address frequencies up to 1000 GHz. Details of MPM are given in two Appendixes. 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.

Keywords: atmospheric attenuation and delay; propagation program MPM; laboratory studies of moist air attenuation; millimeter/submillimeter-wave spectral range; radio path data

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Michael G. Cotton
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