298 BELL SYSTEM TECHNICAL JOURNAL 



Electric Co. of 10 cm. magnetrons under 200 kw. output power. 



The first unstrapped magnetrons coded for manufacture by the Western 

 Electric Co. were the 706A-C for frequencies near 3060 mc/s (9.8 cm.). 

 The frequency differences were achieved by small changes in the resonator 

 slot widths. Another magnetron, the 714A, oscillating at 3300 mc/s (9.1 

 cm.) was also coded at this time. It differed from the others in having 

 slightly smaller resonator holes. These magnetrons were used in some of the 

 earliest American ship and airborne radar systems. A set of typical operat- 

 ing data is included in Table II. 



The improvement in measured over-all efficiency to approximately 25 

 per cent was undoubtedly the result of improvements in the technique of 

 operation and of measuring the output power. Unlike the measurement of 

 frequency, for which good techniques were already available, the measure- 

 ment of output power initially was crude and unreliable. At first, esti- 

 mates of power were based on the heating of resistors to incandescence, with 

 no assurance that all the power was being dissipated in this load. The 

 production of corona has always been spectacular evidence of RF power. 

 Later, when water load techniques were used, all the power could be ab- 

 sorbed and measured. Means of line tuning permitted the adjustment of 

 the magnetron load impedance to the point of maximum output power even 

 though the impedance itself was unknown. 



Some preliminary studies of magnetron tuning were made as auxiliary 

 experiments. With one or more extra coupling loops terminated externally 

 in adjustable coaxial tuners, tuning ranges of 2 to 4 per cent were achieved. 



15.2 The 706AY-GY, 7 MAY, and 718AY-EY Magnetrons: A signifi- 

 cant improvement in the multiresonator magnetron was the method of 

 strapping, used by the British as a means of achieving mode frequency 

 separation (see discussion in PART I). When used in the resonator system 

 of the 706A-C magnetrons, the early British "mode locking straps" in- 

 creased the TT mode wavelength from 9.8 cm. to 10.7 cm. and resulted in an 

 increase in operating efficiency by about a factor of two with much more 

 freedom from "moding" difficulties. Here was a really worthwhile advance 

 and no time was lost in making use of it. The resultant strapped magnetron, 

 oscillating at 10.7 cm. wavelength, was used directly as the basis for a new 

 family of magnetrons of wavelength near this value. These were coded as 

 the 718AY-EY series. The cathode, output circuit, and general mechan- 

 ical features of the 706A-C were adopted essentially without change. Ex- 

 periments like those done on the original British design, when repeated on 

 strapped magnetrons, indicated the design parameters still to be close to 

 optimum. The cathodes used were plain, oxide coated, nickel cylinders. 



