180 



DIELECTRIC COxNSTANT. ABSORPTION AND SCATTERING 



because of tlie ovograpliiu cliaraelev of the preeipiia- 

 tion, wliicli was made up of drops falling on the aver- 

 age uot iiinch more than 1,000 ft, as compared to ordi- 

 nary rain falling 5,000 to 6,000 ft. The longer period 

 of fall in ordinary rain probably permits a greater 

 number of drops to reach the characteristic sizes. 



10.5 ABSORPTION OF MICROWAVES BY 

 THE ATMOSPHERE, BRITISH WORK' 



The working committee of the Ultra Short Wave 

 Panel has presented a report in which the following 

 questions are treated in detail : 



1. The absorption of microwave radiation l)y oxy- 

 gen and vapor. 



2. The absorption of microwave radiation by water 

 iu macroscopic form, for example, rain, clouds, fog. 



3. The measurement in the laboratory of dielectric 

 constants and conductivity which have already been 

 described (see Section 9.3). 



The work under item (1) consisted chiefly in show- 

 ing that tiie attenuation given l:>y Telecommunication 

 Research Establishment radar experiments at Llan- 

 dudno at a wavelength of 1.25 cm was consistent with 

 the theoretical values given in reference 1. The experi- 

 mental method consisted of comparing the echoes re- 

 ceived on X- and K-band radars from a standard re- 

 flector at short range and from land echoes at a large 

 range. Care was taken to insure that the reflecting tar- 

 gets did not reflect an amount of energy which was 

 dependent on frequency. In this way a mininuim vahie 

 for the attenuation on A = 1.25 em was found to be 

 0.14 clb per kilometer. New values based upon revised 

 values of the widths of the various lines of water vapor 

 and oxygen show that almost the whole of this atten- 

 uation must be due to absorption by water vapor and 

 further that to obtain an alisorption as high as 0.14 

 db per kilometer the frequency at the important water 

 vapor line must lie close to 1.25 cm. 



The work under item (2) has been carried out, in 

 the case of rain, by assuming what seemed a plausible 

 distribution of drop sizes and calculating on this basis 

 the attenuation that would occur at a standai'd pre- 

 cipitation rate. Then Ijy making a clinurtological sur- 

 vey, information can be given of the proportion of 

 time during which the attenuation can be expected 

 to exceed a given value for a radar or radio commu- 

 nication set working on a given wavelength at a given 



'By F. Hoyle, UUra Short Wave Panel, Ministry of Supply, 

 England. 



Jocalioii. The rak-uhitions for a standard precipita- 

 tioii late ga\e the following estimates for the maxi- 

 mum attenuation likely to occur (that is, for the 

 most nnfa\orable drop size distribution that was 

 thought likely to occur). 



S l)and : 0.003 db per km per mm per hr. 



X band : 0.06 db per km per mm per hr. 



K band : 0.22 db per km per mm per hr. 



The clinuitological part of the program involves a 

 great deal of statistical work and is not yet complete. 

 The following preliminary results can be given. 



At Padang in Sumatra we may expect, twelve times 

 a year, periods of 1 hr when the average attenua- 

 tion on 1.25 cm will be some 6 to 12 db per kilometer; 

 on 3.0 cm, 2 to 4 db per kilometer; and on 10 cm, 0.1 

 to 0.2 db per kilometer. 



In England only once a year will the average at- 

 tenuation over a period of one hi.uir reach 1 to 2.5 db 

 per kilometer on 1.25 cm; 0.3 to 0.8 per kilometer 

 on 3.0 cm; and 0.02 to 0.04 db per kilometer on 

 10.0 cm. 



It should be noticed that these attenuation figures 

 are for point-to-point communication and must be 

 doubled in the radar case. 



i«6 DIELECTRIC CONSTANT AND LOSS 

 FACTOR OF LIQUID WATER AND 

 THE ATMOSPHERE^ 



In ])ropagation problems the knowledge of the 

 electric ju-operties of the ground, sea, and fresh water, 

 as well as those of the atnu)S])]ieric gases, are of funda- 

 mental importance. Collected here are the available 

 data on these materials. Clearly, the study of the 

 reflection coefficients leads indirectly to the dielectric 

 properties of these materials. Here we will lie con- 

 cerned more with the direct determination of their 

 dielectric constants and loss factors. 



10.6.1 Experimental Methods 



lllOFJ.Et:T10N-T](ANSMISSION METHOD 



First we should like to sketch here the basis of the 

 dift'erent experimental methods used iu the determina- 

 tion of the dielectric properties of materials. 



One of these is the reflection-transmission method 

 which was used by Ford" in his studies of the prop- 

 erties of the ground. This sanu^ method has also been 

 used recently by Saxton^^'^^ on water in investigating 



°By L. Goldstein, Columbia University Wave Propagation 

 Group. 



