10 BELL SYSTEM TECHNICAL JOURNAL 



band amplifying systems capable of delivering high power without dis- 

 tortion. Commercial success has already been achieved with small num- 

 bers of channels in the Holland-East Indies and the United States-England 

 single-sideband systems. More recently, by applying the principles of 

 negative feedback, Bell Laboratories engineers have developed a short- 

 wave transmitting amplifier of 200 kw capable of handling 12 or more 

 closely spaced telephone channels. 



One might visualize the broad-band transatlantic radio telephone system 

 of the future as being built up of successive groups of these 12-channel 

 blocks. The number of groups that might be used simultaneously is, of 

 course, limited. Over any path where radio transmission depends upon 

 reflections between the ionosphere and the earth. Nature sets a rather 

 definite limit on the range of frequencies that is usable at any given time. 

 In effect, there is provided a transmission path between transmitter and 

 receiver which is capable of passing a broad but nevertheless limited band 

 of frequencies. Frequencies above this band are not consistently returned 

 to earth from the ionized regions. Frequencies below this range are 

 absorbed. The high-frequency end is marked by a sharp cutoff, while 

 there is a more gradual diminution of effectiveness at the low-frequency end. 

 The position in the spectrum of the useful band shifts with time of day, 

 season of the year and phase of the solar cycle. Its width varies, too, 

 being narrow at night and wider during the day. 



Thus, for example, when the sun is over the mid-Atlantic in summer 

 there is available a useful band of frequencies about 4 megacycles wide, 

 extending from about 14 to 18 megacycles. It is not sharply defined on 

 its lower side, and its position in the spectrum varies with the season of 

 the year and the sunspot cycle. But ^\e may say roughly that nature 

 provides at any one time, at least during the most useful hours of the day, 

 a band width of the order of 4 megacycles. If this entire range could be 

 utilized for telephony over this particular path, and were subdivided sharply 

 into telephone bands of 4 kilocycles width, there could be realized 1,000 

 telephone channels. These might be used in any of the several ways, as 

 to two-way transmission and as to the points at which they terminate. 



But public service transatlantic telephony is not the only service requiring 

 these important short waves. There are many other uses of them such as 

 radio telegraphy, ship-to-shore telephony, airplane communication and 

 navigation, and overseas broadcasting. There are also other natural bar- 

 riers than the Atlantic to be bridged in this manner, and these short waves 

 because of their world-wide effect and despite the directivity that can be 

 imparted to them, cannot be counted upon to be duplicated very often 

 for simultaneous use at different locations throughout the world. So we 

 must allow for the available 4 megacycles to be divided to meet a large 



