320 



BELL SYSTEM TECHNICAL JOURNAL 



From the inception of the project until the bulk of the work of developing 

 the tunable 3 cm. magnetron was transferred to the Bell Laboratories, a 

 number of problems had been encountered and solved at the Columbia 

 Laboratory. It had been shown that tuning a 725A type magnetron was 



^M^ 



Fig. 71 — A view of a sectioned 2J51, tunable, "packaged" magnetron (55 k\v., 8500 

 to 9600 mc/s). Features of interest to he seen are: the tuning pins, the drive mechanism 

 and vacuum bellows, the axial cathode mount, the glass bead support of the center con- 

 ductor of the coaxial output line, and the sheathed permanent magnet construction. 



feasible by the means chosen. The range of wavelength from 3.13 to 3.53 

 cm. could easily be spanned. The variation in frequency was found to be 

 very nearly linear with position of the tuning pins. It was shown how 

 a spurious resonance of the pin structure occurring within the frequency 

 range could be displaced so as to cause no difliculty. Although no work of 

 revision had been done on the 725A output circuit used in the tunable 

 magnetron, it was demonstrated that tolerably "flat" characteristics of 



