Institute for Telecommunication Sciences
the research laboratory of the National Telecommunications and Information Administration

Hans J. Liebe; Donald H. Layton

Abstract: Laboratory measurements have been performed at 138 GHz of water vapor attenuation ax for pure vapor (H20) and its mixtures with air, nitrogen (N2), oxygen (O2), and Argon (Ar). Temperatures ranged from 8 to 43°C, relatlve humidities from 0 to 95% and total pressures reached 1.5 atm. A computer-contro lled resonance spectrometer was employed. The results are interpreted in terms of underlying absorption mechanisms. Broadening efficiencies m of mixtures H20 + N2, 02, Ar agree among themselves with those measured within cores of the 22 and 183 GHz H20 absorption 1ines. The m-factors are app1ied to predict what share al of the total ax results from the complete pressure broadened H20 spectrum. A substantial amount of the selfbroadening term proportional to the square of vapor pressure is left unaccounted. The negative temperature coefficient of the excess absorption is consistent with a dimer (H20) 2 mode 1. An empirical formulation of the experimental findings is incorporated into the parametric propagation model MPM that utilizes a local (30x H20, 48x 02) line base to address frequencies up to 1000 GHz. Details of MPM are given in two Appendixes. Predictions of moist air attenuation and delay 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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