K-BAND ATTENUATION DUE TO RAINFALL 



177 



0.05 



~ao3 



1.2 1.4 1.6 



XIN CM 



Figure 19. Attenuation coefficient, water vaiior, 45 C. 



of water vapor pef cul)ic meter for humidities of 10 g 

 per cubic meter and of 50 g per cubic meter. It is to 

 be noted that the peak of this curve, at 1.33 cm, is 

 xevy close to tlic standard K-band wavelength. 



These experimental results are in agreement with 

 other results for the water vapor attenuation at K 

 band. Furthermore, for all practical radar purposes, 

 and within the range of the measurements, they are 

 in agreement with Van Aleck's theory of the absorp- 

 tion of this water vapor line. 



Discussion 



Comments were made on the great accuracy of the 

 experiment, pointing out that the c^uantity being 

 measured was extremely small and that other experi- 

 ments, particularly one made in Florida by measure- 

 ment of the sky temperature, were leading to substan- 

 tial agreement with the present findings. The best 

 available data on the performance of K-band radars 

 supported the experimental result obtained and would 

 be of great help in choosing wavelengths for radar and 

 other apparatus in the future. 



The good agreement between theory and the experi- 

 mental result achieved was stressed. Whereas in infra- 

 red absorption measurements the results had disagreed 



witJi tlieory by as much as 10 dli or more, the discrep- 

 ancy in these results amounted to a few per cent only. 

 It was noted that from the purely physical standpoint 

 this water vapor line and the 0.5-cm oxygen absorption 

 line were the most carefully investigated lines in the 

 siJeetnim, aside from some lines in the visible region. 

 The explanation was given that the microwave meas- 

 urements were much more instructive than the optical 

 ones from the standpoint of the collision broadening 

 theory because the width of the microwave absorption 

 line was comparable to the frequency of the radiation. 

 This results in a shape factor or line form which can 

 be studied in detail. The reported dependence on den- 

 sity presents a difficult problem which is not yet well 

 understood. 



The practical importance of the curvature shown in 

 Figure 18 was emphasized. It was pointed out that 

 the effects of a wide range of humidities had been in- 

 vestigated, some very high compared to those ordi- 

 narily encountered. In practical work most of the data 

 Would be obtained from the low end of the curves of 

 Figure 18, where little amljiguity in numerical values 

 would obtain. 



10 * K-BAND ATTENUATION DUE TO 

 RAINFALL" 



^"■*^ Introduction 



In order to determine the attenuation of 1.25-cm 

 wavelength radiation by rain, controlled radio and 

 meteorological measurements were undertaken in an 

 area providing adequate climatic conditions for the 

 study. It was apparent that the attenuation measure- 

 ments should be made in an area of maximum precipi- 

 tation for expediency. Furthermore, the experiment 

 demanded periods of varying rates of rainfall with fre- 

 quent "clearing" for calibration purposes. Tropical 

 orographic (moitntainous) rain seemed to ofEer the 

 greatest probability of fulfilling these conditions. 



A brief reconnaissance of the Hilo, Hawaii, area 

 showed that a site near Kaumana was adequate, hav- 

 ing a yearly fall in excess of 250 in., as compared with 

 an annual rainfall of 10.10 in. in the San Diego area. 

 A 1.31 statute mile link was chosen parallel to the 

 mean trade wind vector, i.e., due east-west, and was 

 located on a lava flow of 1881. The lava was covered 

 with saw grass and low brush. The terrain had a gentle 

 slope from the receiver at 3,500 ft to the transmitter 

 at 3,800 ft above mean sea level. 



PBy L. J. Anderson, U. S. Navy Radio and Sound Lab- 

 oratory. 



