MAGNETRON AS GENERATOR OF CENTIMETER WAVES 205 



exhibits this kind of behavior. In general, it would be preferable to operate 

 this magnetron at magnetic fields above the "valley," but considerations 

 having to do with available magnetic fields, in some cases may make it 

 necessary to operate near the efficiency maximum at lower magnetic fields. 



5. The RF Circuit of the Magnetron Oscillator 



5.1 General Considerations: The discussion to this point has presented a 

 picture of how the multicavity magnetron oscillator works from the point 

 of view of its electronics; how in this respect it is related to other types of 

 magnetron oscillators; and how, on the basis of the picture of the electronic 

 mechanism, some of its fundamental operating characteristics are to be ac- 

 counted for. In the same manner the RF circuit of the magnetron oscilla- 

 tor, comprising the resonator system, the output circuit, and the load, will 

 be discussed. The importance of this part of the magnetron is apparent. 

 It provides the RF fields with which the electrons interact. To do this, 

 electromagnetic energy must be stored in the cavities of the resonator, which 

 reservoir in turn is tapped to deliver energy through the output circuit to 

 the useful load. The detailed manner in which these functions are per- 

 formed has a bearing, not only on such circuit characteristics as circuit 

 efl&ciency or on the effect of load on frequency, but, as has been seen, on the 

 electronic efficiency as well. Furthermore, the circuit analysis of the mag- 

 netron oscillator enables one to explain the remaining important operating 

 characteristic, the so-called Rieke diagram, which describes the operational 

 dependence on load. 



The type of resonator system used in the magnetron oscillator of concern 

 here has already been indicated in Fig. 1. It is a resonator system com- 

 prising a number of cavities spaced equally about the cylindrical anode. 

 This general shape is dictated by the slotted anode cylinder upon which the 

 RF interaction field is set up. To be sure, other types of resonators have 

 been devised which contrive to place tt mode potentials on the anode seg- 

 ments of a cylindrical magnetron. Here, however, except for brief refer- 

 ences, the discussion will concern itself with the multicavity resonator sys- 

 tem of the general type shown in Fig. 1. Although the individual cavities 

 have not been limited to hole and slot geometry like those shown in Fig. 1, 

 and other features have been added, a resonator system consisting of a sys- 

 tem of cavities, arranged around the anode in the manner of Fig. 1, has been 

 used in the majority of magnetron oscillators developed for centimeter 

 wave generation since 1940. 



5.2 Simple Single Frequency Resonator: The fact that the magnetron 

 resonator system has a number of cavities electromagnetically coupled to- 

 gether makes it multiresonant. What has been learned about the various 

 modes and their electromagnetic field configurations, and how they may in 

 a sense be controlled to improve magnetron operation must be discussed in 

 some detail. Before this is done, however, it would be well to refresh one's 

 memory as to the fundamental ideas concerned with a single electromagnetic 



