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BELL SYSTEM TECHNICAL JOURNAL 



change of diameter of the coaxial center conductor at a critical distance from 

 the loop terminals is to be seen in Fig. 66. The height of the output loop 

 above the resonator end surface was a convenient variable upon which the 

 over-all output coupling depended. This was not without disadvantages, 

 however, since its value for reproducibility had to be held to within 0.001 in. 

 Its nominal height above the anode is 0.011 in. 



While the scaling of 10 cm. magnetrons to 3 cm. demanded higher mag- 

 netic fields than had previously been used at 3 cm., the magnet size was 



Fig. 65 — An external view of the 725A magnetron (55 kw., 9375 mc/s), showing the 

 attached coaxial to wave guide junction in the output circuit. 



kept down by the reduction of the magnet gap made possible in the scaling 

 of anode length. A further reduction in effective magnet gap was made by 

 the inclusion of steel disks in the end covers of the magnetron. With these 

 changes, the magnetomotive force required to supply the field for the 725A 

 was almost identical to that required by the 2J21 and no extensive magnet 

 redesign was necessar}^ 



The requirement of interchangeability with the 2J21 made it necessar}- to 

 place the input and output leads of the 725 A at right angles (see Fig. 65), 

 Along with the 725A, there was developed a version, coded the 730A, with 

 input leads mounted directly opposite to the output in a so-called "straight 



