WORLD-WIDE TELEPHONY 497 



also because of the difficulty of obtaining a high degree of directivity. 

 Advantage has been obtained in the transatlantic long-wave channel 

 by locating directive receiving antennas in the most northerly portions 

 of the United States and Great Britain. 



Short-Wave Radio 



The short-wave range, as indicated in Fig. 7, covers a very much 

 broader band of frequencies and offers hope for a much larger number 

 of circuits. Only a portion of the entire range can be used at any one 

 time for circuits between any two points because the distance at which 

 the refracted waves reach the earth's surface varies with the frequency. 

 It also varies with the time of day and the season of year, making it 

 necessary, in general, to have about three different wave-lengths 

 allocated for each circuit. In view of this limitation, it appears that 

 this range can provide theoretically perhaps 50 telephone circuits 

 between two given points. This theoretical possibility is reduced by 

 practical considerations and by the necessity of sharing the range with 

 other radio services. 



Due, however, to the fact that short waves are more restricted 

 than long weaves in distance and in the time of day for which they are 

 effective, it may be possible to use the same frequency for different 

 circuits in several parts of the world without interference. It seems 

 now that the development of the present art may make available 

 throughout the entire world short-wave intercontinental circuits 

 numbered in the hundreds but not in the thousands. This is not an 

 inexhaustible supply of channels for the world to share as is illustrated 

 by the very much larger number of telephone circuits already required 

 for long distance service in the great continental telephone networks. 

 In the United States alone there are about 6500 circuits in long dis- 

 tance service of which 620 are over 600 miles (1000 kilometers) in length. 



The fact that by proper selection of frequency it is possible to 

 communicate over the longest distances, gives short waves a flexibility 

 w^hich is unique. They give particularly good results in routes 

 crossing the equator, such as circuits between Europe or North 

 America and South America, for which routes long-wave transmission 

 would be impracticable because of heavy atmospheric disturbances. 

 On the other hand, short-wave transmission is subject to variations 

 and interruptions from fading, distortion, and effects connected with 

 terrestrial magnetic disturbances. These effects appear to be particu- 

 larly severe on circuits involving routes of transit near the polar 

 regions, such as the route between North America and Europe. 

 Some of these effects are illustrated in Figs. 8 and 9 which give data 



