598 THE GENERATION OF MICROWAVE POWER 



has yet reached operational service, but the advantages that they offer 

 will very probably lead to operational use at some future date. 



The traveling wave tube is a beam-type amplifier as is the klystron. The 

 high-power versions can be applied in systems in the same manner as 

 klystron amplifiers. The most significant advantage that traveling wave 

 tubes can offer is bandwidth. Whereas high-power pulse klystrons have 

 demonstrated bandwidths as great as 5 per cent of the center frequency and 

 can undoubtedly be designed for even wider bands, the traveling wave tube 

 has greater ultimate capabilities. The wide bandwidth can be of substantial 

 value in systems that employ frequency diversity as a means of combatting 

 countermeasures. With wide bandwidth it is also possible to design 

 advanced types of systems for distinguishing moving targets from ground 

 clutter when the radar is mounted on a moving platform. 



Principles of Operation. A schematic diagram of a traveling wave 

 tube is shown in Fig. 11-24. An electron beam is formed by a cathode and 



Signal 

 Input 



Signal 

 Output 



Cathode 



■Slow Wave Structure 



D 



z-I] 



'-Electron Bear 



^Attenuator y 



-Anode ^Collector 



Fig. 11-24 Simplified Diagram Showing Basic Parts of a Traveling-Wave Tube. 



anode structure. After passing through an aperture in the anode, the 

 electron beam travels down the axis of a slow wave structure, shown 

 symbolically as a helix. The slow wave structure is so designed that an 

 electromagnetic wave propagates along it at a relatively slow velocity, 

 a velocity nearly synchronous with that of the electron beam. When 

 the velocity of the beam is nearly synchronous with the velocity of the 

 propagating electromagnetic wave, the electromagnetic fields associated 

 with the beam continuously interact with the propagating wave on the 

 slow wave structure. As a result of this interaction, there is transfer of 

 energy from the electron beam to the propagating electromagnetic wave. 

 A wave that is started by a signal injected at the input end is thereby 

 amplified and emerges from the output with greater signal strength. 



A wave that is reflected from a mismatch in the output circuit will travel 

 in the opposite direction along the slow wave structure toward the input. 

 The feedback of energy in this reflected wave may well be sufficient to cause 

 oscillations if additional means are not incorporated in the tube to suppress 



