272 BELL SYSTEM TECHNICAL JOURNAL 



life expectancy. The modulator of the radar set provided pulsed input 

 power to the oscillator at about 12 kv. and up to 10 amps, peak current.^^ 



The performance of the 10 cm. multicavity magnetrons appeared to 

 make the development of such a generator at 40 cm. feasible. A straight- 

 forward enlargement of the 10 cm. magnetron by a factor of four was out 

 of the question, however, as it resulted in a magnetron entirely too bulky, 

 requiring a prohibitively large magnet. The development of the 700 mc/s 

 magnetron oscillator thus involved departures from the British design. 

 In particular it was found necessary to reduce the axial length of the reso- 

 nator system to a considerably smaller fraction of a wavelength than in the 

 10 cm. design. The development involved design of the interaction space 

 for maximum operating efficiency, the resonator system, for which both 

 eight and sLx resonator structures were employed, and the output circuit 

 for coupling into the existing radar system. 



An early 700 mc/s multicavity magnetron design employed eight reso- 

 nators of axial length less than one tenth wavelength; the 10 cm. design 

 was about one fifth wavelength long. Operating models initially produced 

 approximately 10 kw. of RF power near the desired frequency. It was 

 found, however, that a smaller and lighter magnetron could be made to 

 operate at the same voltage if the number of resonators were reduced from 

 eight to six, permitting smaller anode and cathode radii [equation (16) in 

 PART I]. The weight and over-all diameter was further r-educed by use of 

 elongated holes in the hole and slot resonators. This change resulted in the 

 resonator system used in the 700A-D magnetrons (see Figs. 41 and 46). 

 Each hole is made by boring two intersecting cylinders in the resonator block 

 as may be seen in Fig. 46. No difficulty was encountered in achieving the 

 desired frequency. The frequency differences between the four coded 

 magnetrons near 700 mc/s were achieved by variation of the resonator slot 

 width. 



The separation of mode frequency between the n = 3 mode (tt mode) and 

 the nearest other mode is of the order of 3 per cent. Although this is small 

 compared to that obtainable in strapped magnetrons, it is greater than that 

 for the early unstrapped magnetrons near 10 cm. This is reflected in greater 

 operating efficiency. 



The cathode in the 700A-D magnetrons is supported, as in the British 

 magnetron, by radial leads extending across the center of one of the hole 

 and slot resonators. The cathode diameter was varied in an experiment 

 designed to determine the value for maximum operating efficiency. Early 

 experiments of this type involving measurements of output power and effi- 

 ciency were quite crude and conclusions from their results were by no means 

 as significant as those based on measurements of frequency. The primary 



" This radar development is discussed in: VV. C. Tinus and W. H. C. Higgins, "Early 

 Fire Control Radar for Naval Vessels," Bell Syst. Tech. Jour., 25, 1 (1946). 



