11-4] MODULATION TECHNIQUES FOR BEAM-TYPE AMPLIFIERS 599 



this reflected wave. An attenuator of some sort must generally be intro- 

 duced near the midpoint of this slow wave structure to absorb the reflected 

 wave if a high-gain traveling wave tube is to be a stable amplifier. The 

 wave traveling in the forward direction along the slow wave structure is 

 also attenuated by the attenuator, but the signal information is carried 

 forward through the attenuator by the modulated electron beam, which 

 re-induces a traveling wave in the slow wave structure on the output side 

 of the attenuator. Proper design of the attenuator is one of the most 

 difficult problems in designing a high-power traveling wave tube. 



A helix, which is shown symbolically in Fig. 11-24 as the slow wave 

 structure, is actually used as the physical slow wave structure in low-power 

 traveling wave tubes. It is not suitable for high-power tubes because its 

 ability to dissipate heat is very limited and because the interaction between 

 the electron beam and the traveling wave on a helix falls off at the very high 

 voltages necessary for very high power. Other types of structures must be 

 used, generally heavy metallic structures that are capable of dissipating 

 large amounts of heat. Many such structures have been suggested, a large 

 number of them being forms of loaded waveguide with the loading designed 

 to reduce the phase velocity of the traveling wave tube in the waveguide 

 structure. Structures of this class generally have useful bandwidths much 

 less than the simple helix, but still large enough to be very interesting to 

 system designers. Very high-power traveling wave tubes with bandwidths 

 of 10 per cent or greater appear capable of attainment in the near future. 

 These tubes are linear amplifiers and have demonstrated reasonably high 

 efficiencies and stable gains of 30 db and more. 



11-4 MODULATION TECHNIQUES FOR BEAM -TYPE 

 AMPLIFIERS 



One of the disadvantages of beam-type amplifiers such as the klystron 

 and traveling wave tube as compared with the magnetron is that for the 

 same output power, relatively high voltages are required. When cathode 

 modulation is used, design of the modulator is usually more difficult and 

 breakdown problems associated with high operating voltages are aggra- 

 vated. For some applications it is advantageous to introduce an additional 

 modulating electrode into the tube. The resulting complication in the tube 

 may be more than offset by the simplification of the modulator. 



Modulating electrodes can take several forms. One of these, a mesh-type 

 control grid close to the cathode, is illustrated in Fig. 11-25. If such a 

 control grid is curved to conform with one of the equipotentials in the 

 electron beam, it will have little effect upon the beam shape when the 

 potential of the grid is set at the value of that equipotential. This operating 

 potential is normally positive with respect to the cathode potential. When 



