MAGNETRON AS GENERATOR OF CENTIMETER WAVES 245 



Although this equation was derived for a specific point in the equivalent 

 circuit, it is of general vaUdity at any point in the output circuit or load 

 line of the magnetron, provided the quantity (Qext)o is properly interpreted 

 as the external Q measured at match in the line at the same point. 



10. Special Topics 



10.1 Frequency Stabilization: The degree of stability of the operating 

 frequency of the magnetron to load changes is specified by the external Q, as 

 equation (44) mdicates. The external Q, by equation (23), may be mcreased 

 either by decreasing the load conductance, G[ , or by increasing the circuit 

 characteristic admittance, I'oc- The first alternative may be accomphshed 

 by reduction of the coupling between the load and magnetron resonator sys- 

 tem. Although this results in greater frequency stability, it entails a reduc- 

 tion in output power. Increase of the characteristic admittance of the 

 magnetron resonator system, on the other hand, increases the energy storage 

 capacity as indicated by equation (20) without appreciably changing the 

 output power. Frequency stabihty may be increased in this way either by 

 redesign of the magnetron resonator system or by coupling to it a tuned 

 cavity of high unloaded Q. In the latter case, the degree of stabilization, 

 defined as the ratio of energy stored in the combination of magnetron reso- 

 nator system and stabilizing cavity to the energy stored in the magnetron 

 resonator system alone, is the factor by which the external Q is increased and 

 the pulling figure decreased. In actual practice the stabilizing cavity may 

 be coupled into one of the magnetron resonator cavities or may be built into 

 the output circuit. 



10.2 Frequency Sensitive Loads: In the preceding sections it has been 

 seen how the load admittance, among other parameters, determines the 

 frequency at which the magnetron oscillates. If this load admittance is 

 itself a function of frequency, it may be possible for the condition of oscilla- 

 tion to be satisfied at more than one frequency. This fact makes for an 

 uncertainty of operation, which is to be avoided. Should the load fluctuate 

 for example, as it does in many applications, the oscillation may jump dis- 

 continuously from one frequency to another. If the magnetron is pulsed, 

 it may in certain circumstances oscillate at different frequencies on succes- 

 sive pulses. A tunable magnetron operating into a frequency sensitive load 

 exhibits periodic gaps in its tuning characteristic in which the magnetron 

 cannot be made to operate. 



The discussion here will be limited to the specific type of frequency sensi- 

 tive load consisting of a long line terminated in an admittance, assumed to 

 be frequency insensitive, which differs from the characteristic admittance 

 of the line. The input admittance of such a line is represented by a reflec- 



