V, / for Two Modes 

 of Oscillation 



Desired Operating Point 



V, I for Hard - Tube Modulator 



>-V, / for Line • Type 

 Modulator 



11-1] THE MAGNETRON 587 



As was illustrated in Fig. 11-11, 

 the position of the current-voltage 

 relations of a mode depend upon the 

 load impedance. The undesired 

 modes are at frequencies different 

 from the desired mode. The imped- 

 ance presented to the magnetron at 

 those other frequencies may there- 

 fore determine in part whether the 

 magnetron will operate stably in the 

 desired mode. 



Changes of modes may occur in 

 two different ways. In one form the 



shift from one mode to the other occurs during the pulse. This is known as a 

 mode shift. In the other form the frequency is stable during an individual 

 pulse, but may shift back and forth between modes in a random fashion 

 from one pulse to the next. This is known as a mode skip. The two forms 

 of instability may occur in combination. The mode skip is the more 

 common form. With a mode skip the voltage pulses and current pulses 

 appear as in Fig. 11-13. With a mode shift, the voltage and current are as 

 in Fig. 11-14. Either of these phenomena presents a problem to the 

 magnetron and system designer that requires careful attention to detail 

 to solve. 



Fig. 11-12 Voltage-Current Character- 

 istic of Two Modes of Oscillation in a 

 Magnetron, and the Characteristic 

 Curves of Two Modulators. 



Current 



voltage 



Current 



Fig. 11-13 Voltage and Current Pulses Fig. 11-14 Voltage and Current Pulses 

 with a Mode Skip. with a Mode Shift. 



With a pulsed microwave magnetron, the cathode emission is at a high 

 current density during the pulse. The various types of oxide cathodes that 

 are most commonly used are capable of high peak current for only a limited 

 length of time. If the pulse is too long, the cathode emission will be 

 exhausted and sparking will result. The term sparking refers to the cathode 

 limitation that is characterized by the physical transfer of incandescent 

 particles of oxide coating from the cathode to the anode. Sparking may 

 also be observed as the magnetron nears the end of its life and the emissivity 

 of the cathode is reduced. Much effort has been devoted to cathode design 

 and to improvement of processing techniques in recent years, and sub- 

 stantial improvements in cathode emissivity have resulted. 



