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BELL SYSTEM TECHNICAL JOURNAL 



properties of each magnetron before sealing and pumping was an absolute 

 necessity to insure reproducibility. The use of an iris coupling between 

 the resonator and the wave guide presented a possible solution. However, 

 the iris required would be too large to be placed directly at the back of the 

 resonator. It was proposed to put a hubbed rectangular resonant iris at 

 the back of the resonator and a decoupling iris a half wave length distant 

 in the wave guide. This construction provided a resonant cavity between 

 the two irises in which the storage of energ}^ would provide some degree of 

 frequency stabilization. The structure of this output circuit may be seen 

 in the photograph of a cutaway model in Fig. 77. Both higher efficiency of 



Fig. 76 — An external view of the 3J21 "packaged" magnetron (60 k\v., 24,000 mc/s). 



operation and lower pulling figure than obtained with the earlier circuits 

 resulted. As average values, an increase in over-all efficiency from 24 to 

 28 per cent and a drop in pulling figure from 25 to 18 mc/s may be cited. 



The RF voltage breakdown strength of the wave guide window was 

 marginal. A new design de\-eloped at the Columbia and M. I. T. Labora- 

 tories, incorporating a larger window and "streamlined" wave guide con- 

 tours adjacent to it, was modified slightly and used in the 3J21. 



The magnetic pole gap was made 0.290 in. This was as small as was felt 

 practical. This and other steps were taken in an effort to save as much 

 as possible on magnet weight. 



The mechanical problems connected with the fabrication of the 3J21 



