MAGNETRON AS GENERATOR OF CENTIMETER WAVES 287 



In the strap tuning scheme, use was made of straps of channel or U-shaped 

 cross section, inside which the double rimmed "cookie cutter" element is 

 moved. The tuning arrangement of the 5J26 magnetron is of this type. It 

 is the second of the two types of tuning by variation of capacitance dis- 

 cussed in PART I. 



Quite early in this program of development it became apparent that the 

 armed services would require all these tunable magnetrons to be mechanical 

 and electrical replacements for e.xisting ILxed frequency magnetrons, inter- 

 changeable in all respects. This requirement, in addition to the facts of 

 electrical design, indicated that the "cookie cutter" type of tuner repre- 

 sented the line of attack most likely to succeed. The capacitance in the 

 anode structure or in the straps is the frequency determining circuit param- 

 eter most easily varied, as it is the most accessible in these large size mag- 

 netrons. The tuning element may be moved conveniently by a mechanism, 

 compact in the axial dimension, mounted in the end space and cover of the 

 magnetron. These are factors of prime importance in "unpackaged" 

 magnetrons where over-all axial length is important. IMagnetrons in this 

 wavelength range are sufficiently large to make such a mechanical design 

 feasible. 



The magnetrons of wavelength 20 to 45 cm. for which tunable replace- 

 ments were designed were: the 700A-D series (680-720 mc/s), the 728A-J 

 series (900-970 mc/s), and the 4J21-30 series (1220-1350 mc/s). Although 

 it was preferable to span the 4J21-30 band with one tunable replacement, the 

 use of two tunable replacements, each covering half the band, would have 

 been satisfactory had it been necessary. In the development of each of the 

 tunable replacements, improvements and modernizations which could be 

 incorporated within the limitations of mechanical and electrical interchange- 

 ability were made. 



14.2 The 4J42 Magnetron: The tunable replacement for the 700A-D 

 series is the 4J42. It covers an increased frequency band of 660 to 730 mc/s. 

 The tuning is accomplished by variation of the intersegment capacitance of 

 the anode structure itself, the first of the two capacitance tuning methods 

 listed above. Frequency varies nearly linearly with displacement of the 

 tuning member. The tuning range, although adequate for the purposes of 

 the 4J42, is limited by interference of other modes as was discussed in PART 

 I. In this respect it is not a great deal better than can be attained by un- 

 symmetrical means. The advantages of symmetrical capacitance tuning 

 enumerated above made it the logical choice nevertheless. A satisfactory 

 tuning range of the tt mode without interference from other modes was 

 achieved by strapping the end of the magnetron opposite that at which the 

 tuning mechanism is mounted. This necessitated changes in the resonator 



