TRAVELING-WAVE TUBES 3 



gence by telegraph, by telephone or by facsimile is directly proportional 

 to bandwidth; and, with an increase in communication in all of these fields, 

 more bandwidth is needed. 



Further, new services require much more bandwidth than old services. 

 A bandwidth of 4,000 cycles suffices for a telephone conversation. A band- 

 width of 15,000 cycles is required for a very-high-fidelity program circuit. 

 A single black-and-white television channel occupies a bandwidth of about 

 4 mc, or approximately a thousand times the bandwidth required for te- 

 lephony. 



Beyond these requirements for greater bandwidth to transmit greater 

 amounts of intelligence and to provide new types of service, there is cur- 

 rently a third need for more bandwidth. In FM broadcasting, a radio fre- 

 quency bandwidth of 150 kc is used in transmitting a 15 kc audio channel. 

 This ten-fold increase in bandwidth does not represent a waste of frequency 

 space, because by using the extra bandwidth a considerable immunity to 

 noise and interference is achieved. Other attractive types of modulation, 

 such as PCM (pulse code modulation) also make use of wide bandwidths 

 in overcoming distortion, noise and interference. 



At present, the media of communication which have been used in the past 

 are becoming increasingly crowded. With a bandwidth of about 3 mc, 

 approximately 600 telephone channels can be transmitted on a single coaxial 

 cable. It is very hard to make amplifiers which have the high quality neces- 

 sary for single sideband transmission with bandwidths more than a few times 

 broader than this. In television there are a number of channels suitable for 

 local broadcasting in the range around 100 mc, and amplifiers sufficiently 

 broad and of sufficiently good quality to amphfy a single television channel 

 for a small number of times are available. It is clear, however, that at these 

 lower frequencies it would be very difficult to provide a number of long-haul 

 television channels and to increase telephone and other services substan- 

 tially. 



Fortunately, the microwave spectrum, wh'ch has been exploited increas- 

 ingly since the war, provides a great deal of new frequency space. For in- 

 stance, the entire broadcast band, which is about 1 mc wide, is not sufficient 

 for one television signal. The small part of the microwave spectrum in the 

 [ wavelength range from 6 to 7^ cm has a frequency range of 1,000 mc, which 

 ' is sufficient to transmit many simultaneous television channels, even when 

 broad-band methods such as FM or PCM are used. 



In order fully to exploit the microwave spectrum, it is desirable to have 

 ! amplifiers with bandwidths commensurate with the frequency space avail- 

 i able. This is partly because one wishes to send a great deal of information 

 in the microwave range: a great many telephone channels and a substan- 

 tial number of television channels. There is another reason why very broad 



