MAGNETRON AS GENERATOR OF CENTIMETER WAVES 305 



The five wavelength groups of the 720A-E employ straps of different 

 height. 



The cathode was similar to that of the 706AY-GY but longer and with 

 different end disk design. The surface was a plain oxide coating on the 

 nickel cylinder base. Adequate life under the most stringent of its operating 

 conditions was obtained (see Table II). The radical departure in cathode 

 end disk design was necessitated by the pickup of RF energy by the cathode. 

 This was aggravated by the length of the cathode. An end disk of con- 

 ventional size, being closer to the inner strap than to the outer strap, is more 

 influenced by its potential. Since the inner straps are t radians out of 

 phase and the cathode approximately a half wavelength long, conditions 

 were quite right for considerable induction of RF potential on the cathode. 

 Considerable RF power was radiated by the cathode leads. Measurements 

 on a non-oscillating magnetron indicated that cathode pickup could be 

 neutralized very nearly by increasing the end disk diameter. If the end 

 disk completely covered both straps, the charge induced on it by one strap 

 was essentially balanced by that induced by the other strap. The 720A-E 

 with large cathode end disks radiates very little RF energy from its cathode 

 leads. 



The greater power which the 720A-E produced made it necessary to re- 

 design the output circuit to include the transformer inside the vacuum. 

 Voltages in the external transformer had been sufficiently high to break down 

 the line. The redesign was carried out empirically by a substitution method. 

 A replica of the output circuit was constructed and terminated in the im- 

 pedance required to load the magnetron properly. In this replica the loop 

 was replaced by a coaxial line and standing wave detector through which 

 CW power was fed. In this way one could determine the impedance at the 

 loop terminals necessary for proper loading. Then with a specially con- 

 structed output circuit, in which the dimensions between the glass seal and 

 the loop terminals could be varied, the dimensional changes necessary to 

 achieve the required impedance at the loop with match in the output load 

 coaxial was determined. The dimensions determined in this way provided 

 the final design. The 720A-E was one of the early "preplumbed" 

 magnetrons. 



A coaxial lead coupling using chokes instead of contacts was developed 

 for the output circuit and used not only at 10 cm. but on all the later mag- 

 netrons of longer wavelength as has been described. The centrifugal glass 

 seal at the outer conductor is to be noted (see Fig. 63). The original tung- 

 sten center conductor to which an external "thimble" was soldered (similar 

 to that in the 5J23, see Fig. 49) was replaced by a Kovar cup to which the 

 glass was sealed directly. Greater strength, better cooling, and elimination 



