The Bell System Technical Journal 



Vol. XV January, 1936 No. 1 



Long- Wave Radio Transmission Phenomena Associated 

 with a Cessation of the Sun's Rays 



By AUSTIN BAILEY and A. E. HARPER 



The variations in long- wave radio field strength near the time of sunset 

 on long transmission paths, which have been reported by many observers, 

 were studied for the purpose of formulating rational methods of forecasting 

 their time of occurrence. During some preliminary observations fair agree- 

 ment was found between the time of minimum field and the sun's position 

 relative to a particular point on the transatlantic path under observation. 

 The more extended study of radio field variations during sunset periods and 

 solar eclipses disclosed that in general no exact relationship could be estab- 

 lished between the sun's position at any point and the occurrence of the 

 minimum field. 



Observations of field variations were made on radio signals at a number 

 of diflFerent frequencies and over several paths. It was concluded that a 

 characteristic sunset cycle of field variations is present on frequencies 

 between 18 kc. and 68 kc. for transmission paths longer than 700 krn. 

 For paths less than 200 km. long, such variations are negligible. There is 

 some evidence that the amplitude of these field variations is smaller at lower 

 frequencies. 



Analysis of the data presented indicates that long waves over long 

 paths are transmitted predominately by "sky waves." From the data it 

 was not possible to establish any satisfactory picture of the path followed. 

 It was established, however, that empirical methods based on observations 

 over a particular transmission path may be employed to forecast the 

 approximate time of occurrence of field variations. 



Introduction 



COMMERCIAL transatlantic radio telephone service to Europe was 

 inaugurated on January 7, 1927, using a long-wave circuit on a 

 frequency of 60 kilocycles. The difficulties of maintaining a satis- 

 factory long-wave circuit through that part of each day when sunset 

 conditions existed along the transmission path had been recognized ^ * 

 for several years prior to the opening of service. After short-wave 

 facilities were added to the transatlantic service, it became important 

 to coordinate operating times of the short-wave transmitters in relation 

 to the long-wave channel to assure maximum reliability and efficiency 

 of service. In order to do this, a more precise knowledge of the 



* Numbers refer to appended list of references. 



1 



