496 BELL SYSTEM TECHNICAL JOURNAL 



this reason the very high frequency range is not indicated in Fig. 7 as 

 suitable for intercontinental circuits. Where the route is over-land it 

 is possible that such rays may find practical use in forming links in 

 intercontinental circuits particularly where the topography of the 

 country affords advantageous elevated locations for intermediate re- 

 peater points. Too little is now known of these very short waves to 

 make their discussion other than speculative. 



An important requirement for radio for overseas telephone circuits 

 is the avoidance of overhearing. A number of the overseas circuits 

 now in operation, including the transatlantic group, are equipped 

 with privacy arrangements which so modify the frequency disposition 

 of the voice waves as to prevent overhearing of the conversations by 

 other radio stations not equipped with similar arrangements. Experi- 

 ments have been made with more elaborate arrangements for obtaining 

 an even higher degree of privacy than that now provided. 



Long Wave Circuits 



Experience has shown that good results can be obtained in the 

 long-w-ave range on such circuits as those between North America and 

 Europe which are 3000 miles (5000 kilometers) long and have their trans- 

 mission paths at a high latitude. A fundamental limitation to the 

 extensive use of long waves, however, is that within the range of 

 suitable frequencies there are theoretically obtainable only about 

 twenty telephone channels between any two points. Actually how- 

 ever, there are not this many available because of the practical 

 limitations imposed by the necessity of sharing this range of frequencies 

 with other types of radio service. The relatively low attenuation of 

 these waves as they are transmitted over the earth's surface makes it 

 appear impractical to use duplicate intercontinental circuits of the 

 same frequency at different points on the earth's surface. The total 

 number of circuits of the long-wave type which can form a part of 

 the ultimate world-wide telephone network seems, therefore, to be 

 very small. 



Other limitations in the use of long- wave circuits are that with the 

 amounts of power which it now seems practicable to use (a tube 

 capacity of about 300 kilowatts is used in the transmitter of the 

 present transatlantic long wave circuit) their successful application 

 does not exceed several thousand kilometers and is confined to routes 

 well outside of the equatorial regions. These limitations are necessary 

 to avoid excessive interference from atmospheric disturbances, both 

 because of the relatively large components of these disturbances having 

 frequencies in the range of wave-lengths used by these circuits, and 



