600 



THE GENERATION OF MICROWAVE POWER 



the control grid is sufficiently negative with respect to the cathode, current 

 from the cathode will be cut off. The mu of a structure such as this is 

 relatively high and can be greater than 100. A relatively high-power beam 

 can therefore be switched on and off by relatively small pulses applied to 

 the control grid. A disadvantage of this type of grid is that the grid is 

 heated by its proximity to the cathode and is further heated by electron 

 bombardment during the time the beam is on. If the grid is too hot as a 

 result, there may be electron emission from the grid, which will form a 

 low-current electron beam at a time when the beam should be completely 

 off. An excessive amount of noise or signal may be produced between pulses 

 as a result of this grid emission. 



Cathode 

 Heater 



Focus and Control 

 Electrode 



+ - 



Electron Beam 



Fig. 11-25 High-Mu Mesh-Type Con- 

 trol Grid for an Electron Beam. The 

 Control Grid Is Biased Negative with 

 Respect to the Cathode to Cut Off the 

 Electron Beam, and Is Pulsed Positive 

 with Respect to the Cathode When the 

 Beam is Turned On. 



Fig. 11-26 Low-Mu Control Electrode 

 for an Electron Beam. The Control Grid 

 Is Biased Negative with Respect to the 

 Cathode to Cut Off the Electron Beam, 

 and Is Pulsed up to Cathode Potential 

 when the Beam is Turned On. 



A second type of control electrode, in this instance nonintercepting, is 

 shown in Fig. 11-26. Here the focus electrode adjacent to the cathode, 

 which helps form the electron beam, is insulated from the cathode proper. 

 For proper formation of the electron beam, this control electrode should be 

 operated at cathode potential and will intercept none of the current emitted 

 by the cathode. The beam may be turned off by biasing the control elec- 

 trode negative with respect to the cathode by a voltage sufficient to shield 

 the cathode from the positive anode voltage. A structure of this type has 

 relatively low mu, in the order of 2, but the problem of grid emission can 

 be avoided. 



A third type, sometimes called a modulating anode, is shown in Fig. 

 11-27. Here the anode of the diode which forms the electron beam is 



