MAGNETRON AS GENERATOR OF CENTIMETER WAVES 



339 



magnetron resulted largely from the small tolerances it was found necessary 

 to impose. The tolerance allowed on radial location of the cathode was 

 ±0.002 in.; on transformer slot width (when this output was used), ±0.0005 

 in.; and on dimensions of the anode structure, ±0.001 in. Deformations, 

 such as those produced by placing a magnetron with slightly misaligned pole 

 pieces on a strong magnet, had to be carcfull>' avoided. The importance 



Fig. 77— An internal view of the 3J21 "packaged" magnetron (60 kw., 24,UUU mc, sj. 

 Note the "rising sun" resonator system, the stabilizing cavity in the wave guide output 

 between a rectangular resonant iris at the "back" of the output resonator and a circular 

 decoupling iris a half wavelength distant, and the beveling of the wave guide edges adjacent 

 to the wave guide window. 



of cathode alignment, for example, may be seen from the fact that across the 

 anode-cathode space, which in the 3J21 is only 0.032 in., there is an operat- 

 ing voltage gradient of 165 kv./in. This gradient is doubled in the region 

 where the cathode support passes through the hole in the pole piece, at 

 which point there is a nominal clearance of 0.015 in. Thus the elimination 

 of small burrs and small radii was essential. 



An e.xternal view of the 3J21 magnetron is shown in Fig. 76, an internal 

 view in Fig. 77. Operational and other data are to be found in Table IV. 



