324 BELL SYSTEM TECHNICAL JOURNAL 



Fortunately, before production commenced, relaxation of the interchange- 

 ability requirements made a change in the magnet form possible, and the 

 much more efficient U-shape of those shown in Fig. 70 was adopted. 



In performance, the 2J51 magnetron is essentially a 725 A capable of 

 operating over a 12 per cent frequency band. Table III gives two sets of 

 operating conditions available through the use of the magnetic shunts. Fig. 

 72 shows several circuit characteristics of the magnetron over the frequency 

 band. Also shown is the variation in over-all operating efficiency. The 

 slight drop in efficiency observed as the frequency increases is due primarily 

 to the decrease in Qo. The electronic efficiency increases slightly with 

 increasing frequency. The performance chart of the magnetron at its 

 intermediate magnetic field is very similar to that of the 725A at the same 

 field value, as shown in Fig. 68. The performance of the 2J51, when con- 

 nected to a long, mismatched line exhibits a periodicity in power output, 

 pulling figure, and frequency variation as the tuning is changed. This is 

 expected in such a case because of the resulting changes occurring in the 

 electrical length of the line with changing frequency. If the mismatch is 

 sufficiently great, the condition may exist in which periodic regions of the 

 frequency spectrum are completely unattainable, as discussed in section 10.2 

 Frequency Sensitive Loads. 



The 2J51 represents an attempt to design considerable versatility into one 

 magnetron. As has been described in the discussion of the 2J55 and 2J56, 

 the development of the 2J51 provided the opportunity to make available 

 "packaged" magnetrons of fixed wavelengths near 3 cm. by the omission 

 of the tuning mechanism. 



19. Magnetrons for Wavelengths near 3 Centimeters — 

 100 TO 300 Kilowatts 



19.1. The 4J50, 4J52, and 4J78 Magnetrons: In the later stages of devel- 

 opment of the 725A, it had been demonstrated that a high efficiency scaled 

 magnetron of wavelength near 3 cm. was feasible. It also had become evi- 

 dent that, freed from the hampering restrictions on input power and me- 

 chanical interchangeability, a magnetron of considerably greater output 

 power capabilities could be made. The achievement of a magnetron design 

 capable of delivering at least 200 kw. output power at a duty cycle of 0.001 

 or greater was set as a first objective. This was achieved with a good 

 margin. During its entire course, the program was actively participated 

 in by members of the Radiation Laboratory' at M. I. T. working in residence 

 at the Bell Laboratories. 



The objective of 200 kw. peak output power at the factory test was not 

 considered to be the maximum obtainable from a 3 cm. magnetron but 



