8 BELL SYSTEM TECHNICAL JOURNAL 



present practice for an approximately comparable quality of service is 

 about one-half that of long waves. The wider band provides an oppor- 

 tunity for the service to expand, for although the short-wave band is by 

 no means unlimited in extent and within the last few years has become 

 increasingly congested, much can be accomplished by careful and coor- 

 dinated planning for use, and in any case it is hundreds of times wider than 

 the long-wave band. These advantages have been reflected in the rapid 

 growth of short-wave transoceanic telephony, as indicated by the fact that 

 by the beginning of 1939 there were in service throughout the world about 

 170 important long-distance short-wave telephone circuits, of which five 

 were in regular use between the United States and Europe. There has 

 grown up also a host of short-wave broadcasting channels, the better coor- 

 dination of which has yet to be worked out. 



Certain disadvantages of short-wave transmission must nevertheless be 

 reckoned with. The greatest by far is its susceptibility to complete or 

 partial interruption at certain times, particularly around the maxima of 

 the 11-year sunspot cycle. This weakness of short-wave transmission is 

 fresh in the minds of many of us because we are only now emerging from 

 one of these maxima. Short waves are also affected adversely by various 

 types of signal distortion which arise from the circumstance that the signal 

 picked up at a receiving site is usually made up of several components which 

 have traveled over different paths. Sometimes these paths all lie along 

 the same great circle but involve different numbers of reflections between 

 the earth and the Heaviside layer. Sometimes signal components arrive 

 over other than the great-circle path. Occasionally components travel 

 along the longer of the two great-circle paths between the transmitter and 

 receiver or even clear around the world, producing a distinctive phenomenon 

 known as "round-the-world-echo." Interference between waves arriving 

 over different paths results at times in "general fading" caused by variations 

 in the level of the whole band and at other times in "selective fading" in 

 which portions of the speech band are affected differently. 



These phenomena and their causes have been widely and intensively 

 studied ever since the advent of short waves and as a result much progress 

 has been made in improving short-wave telephone transmission. Single- 

 sideband transmission has been helpful in eliminating a particularly dis- 

 agreeable type of mutilation common to double-sideband transmission 

 wherein the fading out from time to time of the carrier signal gives rise 

 to a harsh, grating character of received speech. The multiple-unit steer- 

 able antenna recently developed in Bell Telephone Laboratories, and known 

 familiarly as "musa," has been found useful in reducing speech distortion 

 accompanying wave-interference effects. The musa reduces selective 

 fading by combining signals arriving over different paths or by eliminating all 



