916 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 195G 



In Fig. 8, measured values are compared with the theory of Van Vleck 

 as calculated by T. F. Rogers using line-breadth constants of 600 mc 

 and 1200 mc per atmosphere. The fit with the 600-mc curve is good from 

 49 to 55.5 kmc, but discrepancies are evident between 56.5 and 59 kmc. 

 For completeness, Rogers' calculations for the absorption at higher alti- 

 tudes are reproduced in Fig. 9. 



A few continuous recordings of rain attenuation have been made at a 

 wavelength of 6.0 mm; a record taken during a light rain is shown in 

 Fig. 10. The median value of the signal is —6.7 db which corresponds to 

 an attenuation of 2.5 db/km for this 5 mm per hour rainfall. During more 

 intensive rainfalls, short-term attenuations in excess of 25 db/km have 

 been observed. 



On one occasion, it was possible to measure attenuation by land fog. 

 The measurements given in Table II were made at a wavelength of 6.0 

 mm. No information regarding water content or drop size was available 

 for this fog. 



CONCLUSION 



A frequency-modulation, two-way transmission technique has proven 

 reliable for measurement of atmospheric attenuation at millimeter wave- 

 lengths. Prerequisite to the success of the method are corner reflectors 

 with good mechanical, thermal and electrical stability. 



The frequency-modulation method has been demonstrated by absorp- 

 tion measurements in the free atmosphere in the 5.1- to 6.1-mm band. 

 The data thus obtained are in good agreement with Van Vleck 's theory 

 of oxygen absorption; the line-breadth constant appropriate to the meas- 

 urements lies between 600 and 800 mc per atmosphere. 



REFERENCES 



1. J. H. Van Vleck, Phys. Rev., 71, pp. 413 ff, 1947. 



2. R. Beringer, Phys. Rev., 70, p. 53, 1946. R. S. Anderson, W. V. Smith and W. 



Gordy, Phys. Rev. 87, p. 561, 1952. J. O. Artman and J. P. Gordon, Phj^s. 

 Rev., 96, p. 1237, 1954. 



3. R. H. Dicke, R. Beringer, R. L. Kyhl, A. B. Vane, Phys. Rev., 70, p. 340, 1946. 



G. E. Mueller, Proc. I.R.E., 34, p. 181, 1946. H. R. Lament, Phys. Rev., 74, 

 p. 353, 1948. 



