586 



THE GENERATION OF MICROWAVE POWER 



shown, the Hartree voltages for other possible modes of oscillation are 

 greater than the cutoff voltage. For any one of the given modes of oscil- 

 lation, the current, voltage relationship will be a function of the external 

 load on the magnetron, as shown in Fig. 11-11. 



-•-7 Modes of 

 X''^ Oscillation 



Hartree 

 Voltage 



I 



Decreasing 



Fig. 11-10 Voltage-Current Relation- 

 ship for Nonoscillating Magnetron and 

 for Two Modes of Oscillation. 



Fig. 11-11 Variation of Voltage-Current 



Relationship with Charging External 



Load on the Magnetron. 



When the magnetron is operated, there is competition between the 

 possible modes of oscillation, and for proper operation the conditions must 

 be such that oscillation occurs only in the desired mode. The conditions 

 are several and they interact. For example, when a pulse is applied to the 

 magnetron, and the leading edge of the pulse reaches the Hartree voltage, 

 a finite time is required for the oscillations to build up. If the leading edge 

 of the pulse is too steep, the oscillations may not build up fast enough for 

 the increase of current to limit the rise in voltage, and the voltage will 

 continue to rise to the point where oscillations start in the next higher 

 voltage mode of oscillation. To prevent this, it is usually necessary to 

 specify the maximum permissible steepness of the leading edge of the 

 voltage pulse that is applied to the magnetron. 



With a rising-sun anode, undesired modes of oscillation can be excited at 

 voltages lower than the voltage of the -k mode. To avoid oscillation in these 

 undesired modes, the leading edge of the voltage pulse cannot be too 

 gradual. 



The voltage-current characteristic of the modulator is also important. 

 Fig. 11-12 shows the voltage-current characteristic of two different types 

 of modulators. As shown, one curve is typical of hard-tube modulators, 

 and the other of line-type pulsers. The two voltage-current characteristics 

 of two modes of oscillation are also shown. It is apparent that with one 

 modulator, the curve intersects both mode curves, and with the other 

 modulator, only one. Oscillation in both modes is therefore possible with 

 the first modulator, but oscillation in only a single mode is possible with 

 the second. 



