4 BELL SYSTEM TECHNICAL JOURNAL 



bands are needed in the microwave range. In providing an integrated nation- 

 wide communication service, it is necessary for the signals to be ampUfied 

 by many repeaters. Amplification of the single-sideband type of signal used 

 in coaxial systems, or even amplification of amplitude modulated signals, 

 requires a freedom from distortion in amplifiers which it seems almost 

 impossible to attain at microwave frequencies, and a freedom from inter- 

 fering signals which it will be very difiicult to attain. For these reasons, it 

 seems almost essential to rely on methods of modulation which use a large 

 bandwidth in order to overcome both amplifier distortion and also inter- 

 ference. 



Many microwave amplifiers are inferior in bandwidth to amplifiers avail- 

 able at lower frequencies. Klystrons give perhaps a little less bandwidth than 

 good low-frequency pentodes. The type 416A triode, recently developed at 

 Bell Telephone Laboratories, gives bandwidths in the 4,000 mc range some- 

 what larger than those attainable at lower frequencies. Both the klystron 

 and the triode have, however, the same fundamental limitation as do other 

 conventional tubes. As the band is broadened at any frequency, the gain is 

 necessarily decreased, and for a given tube there is a bandwidth beyond 

 which no gain is available. This is so because the signal must be applied by 

 means of some sort of resonant circuit across a capacitance at the input of 

 the tube. 



In the traveling-wave tube, this limitation is overcome completely. There 

 is no input capacitance nor any resonant circuit. The tube is a smooth trans- 

 mission line with a negative attenuation in the forward direction and a 

 positive attenuation in the backward direction. The bandwidth can be 

 limited by transducers connecting the circuit of the tube to the source and 

 the load, but the bandwidth of such transducers can be made very great. 

 The tube itself has a gradual change of gain with frequency, and we have seen 

 that this allows a bandwidth of three times and perhaps more. This means 

 that bandwidths of more than 1,000 mc are available in the microwave 

 range. Such bandwidths are indeed so great that at present we have no means 

 for fully exploiting them. 



In all, the traveling-wave tube compares favorably with other microwave 

 devices in gain, in noise figure, in simplicity of construction and in fre- 

 quency range. While it is not as good as the magnetron in efficiency, reason- 

 able efficiencies can be attained and greater efliciencies are to be expected. 

 Finally, it does provide amplification over a bandwidth commensurate with 

 the frequency space available at microwaves. 



The purpose of this book is to collect and present theoretical material 

 which will be useful to those who want to know about, to design or to do 

 research on traveling-wave tubes. Some of this material has appeared in 

 print. Other parts of the material are new. The old material and the new 

 material have been given a common notation. 



