MICROWAVE REPEATER kESEARCE 197 



feet lower in height; sometimes multiple paths were observed with this echo. 

 Space diversity would fail to improve transmission under these propagation 

 conditions, and no other means of improvement is apparent except, perhaps, 

 an alternate path. Fortunately, this type of fading was the least frequent 

 of the three types which were characterized by low signal levels. 



Rain Attenuation and Atmospheric Absorption 



Attenuation effects due to rainfall and absorption by atmospheric gases 

 become increasingly important at the short-wave end of the microwave 

 region. Measurements of rain attenuation have been made at the Holmdel 

 Laboratory^'- ^*; the results are summarized in Fig. 1-5. These curves show 

 that for wavelengths above about 5 centimeters, rain attenuation is not very 

 serious except for rains of cloudburst proportions. However, at wavelengths 

 of one centimeter and less, even moderate rainfall will cause large attenu- 

 ations on paths of the order of 10-20 miles in length. 



Absorption by atmospheric gases, principally water vapor and oxygen, 

 becomes important at wavelengths below about 1 .5 centimeters. According 

 to the theoretical work of Dr. J. H. Van Vleck, Harvard University, water 

 vapor has an absorption band at 1 .?>?> centimeters and oxygen has bands at 

 0.5 and 0.25 centimeters. Measurements made on the Deal-Holmdel path 

 at 1.25 centimeters were in fair agreement with Van Vleck's results and 

 indicated that a typical value of atmospheric absorption for this locality 

 in summertime is about 0.4 db per mile.^^ 



Summary 



In the ultra-short-wave region, transmission has been found to be affected 

 mainly by ground reflections and variable atmospheric refraction; only 

 occasionally are atmospheric reflecting layers and trapping phenomena in- 

 volved. These wavelengths ordinarily are not transmitted to great dis- 

 tances along the surface of the earth, but are diffracted around obstacles. 

 They are used for local broadcasting and mobile radio communication. 



Microwaves are attractive for radio repeater circuits since they permit 

 the use of wide transmission bands. Ground reflections are apparently of 

 small importance with terrain such as that of the Eastern seaboard and sub- 

 stantially free-space propagation is obtained during non-fading periods over 

 optical paths which have approximately "first Fresnel region" clearance. 

 Atmospheric reflecting layers and trapping phenomena are frequently ob- 

 served and signal variations are considerably greater than in the ultra- 



13 Sloan D. Robertson and Archie P. King, "The Effect of Rain upon the Propagation 

 of Waves in the 1- and 3-Centimeter Regions", Proc. I. R. E., vol. 34, pp. 178P-180P; 

 April 1946. 



" G. E. Mueller, "Propagation of 6-Millimeter Waves", Proc. I. R. £., vol. 34, pp. 181P- 

 183P; AprU 1946. 



