286 BELL SYSTEM TECHNICAL JOURNAL 



coaxial line terminated in a plunger or a variable capacitance, or a coaxial 

 to wave guide junction with movable plungers in the wave guide. Another 

 form involved the deformation of one of the resonators itself to accommo- 

 date a prism shaped tuning element which was moved from outside the 

 vacuum through a diaphragm arrangement. 



Although considerable effort was expended in the study of unsymmetrical 

 tuning schemes and much learned about them in the course of this work, 

 they have two rather fundamental drawbacks which caused them to be 

 replaced by the symmetrical types in which these defects could be elimi- 

 nated. In the first place, it is difficult by this means to obtain the desired 

 range of frequency change. Secondly, by virtue of asymmetr}^, operation 

 in other modes is difl&cult to avoid except over rather narrow frequency 

 ranges. Those ty^es in which part of the tuning circuit is outside the 

 vacuum envelope have the additional drawback of bringing high RF volt- 

 ages into structures in air where very high standing waves are necessar>' and 

 breakdown difficult to avoid. 



It was found possible to circumvent each of these difficulties by using 

 symmetrical tuners. Some such tuners were like those tried on 10 cm. 

 models and involved spider-like straps connecting alternate segments to two 

 common points on the axis of the tube. These points in turn were con- 

 nected to the center and outer conductors of a coaxial line. After passing 

 through a vacuum seal, this coaxial line connected to a. reactor like those 

 used in the unsymmetrical schemes. 



In the general arrangement finally used, a tuning member was mounted 

 in the end space. The structure included a vacuum diaphragm mounted 

 in the end cover of the magnetron. The tuning was effected by variation 

 of the capacitance between the resonator system and the tuning member. 



In unstrapped magnetrons, the tuning was accomplished by moving a 

 tuning member of the general shape of a ''cookie cutter" in a single groove 

 cut into the slotted portion of the anode structure. Later, this range was 

 increased by using two slots and a tuning member having two concentric 

 rings. This type of tuner was used on both the 4J42 and 4J51 magnetrons 

 to be discussed. It is the first of the two types of tuning by variation of 

 capacitance discussed in PART I of this paper. Considerable effort was 

 expended to extend the frequency range of tuning by strapping the untuned 

 end of the resonator system and by variations of the resonator dimensions 

 on which relative mode frequencies depend. It was found possible to obtain 

 a tuning range of better than ±5 per cent, but it appears quite difficult 

 to exceed this amount appreciably in high voltage magnetrons using this 

 tuning means. 



