322 BELL SYSTEM TECHNICAL JOURNAL 



with the inductance of the end space region through which the pins project 

 from the magnet pole piece into the resonators. As before, the frequency 

 of this resonance was removed by filling the end space with a copper ring or 

 collar. 



In the course of study of this resonance and of output circuit characteris- 

 tics, an extensive program of testing of non-oscillating experimental models, 

 whether operable or not, was carried out to supplement the data of oscillation 

 tests. From these measurements, three important types of data were 

 obtained, namely, the variation of pulling figure with frequency, the amount 

 of RF energy loss introduced by the insertion of the tuning pins, and the 

 variation of frequency with position of the tuning head. The first of these 

 data, combined with measurements of the output circuit transformer char- 

 acteristics made on simulated and adjustable models, yielded a complete 

 understanding of the functioning of the output circuit. The effect of 

 coupling loop size and of the variation of the dimensions of the coaxial to 

 wave guide junction on the performance of the magnetron over the frequency 

 range were determined. Although other designs of output circuits of the 

 general 725 A type were experimented with, it was possible to adjust the 

 parameters of the original 725A output circuit to give a reasonabl}^ flat 

 characteristic with frequency. In Fig. 72 are shown the variation of pulling 

 figure and Qext over the frequency band. 



The variation of unloaded Q with frequency, obtained in the above men- 

 tioned non-oscillating tests, indicated that stainless steel pins or even such 

 pins copper plated to a considerable thickness were unsatisfactory. It was 

 found that under the heat treatment occurring during brazing operations 

 the copper plate and steel diffused through each other so as to increase 

 markedly the surface resistance of the pins. Copper sheathed pins were 

 found to be satisfactoty. Solid copper was finally used as it was found to 

 possess sufficient strength under mechanical shock test. Even so, as Fig. 

 72 indicates, the Qo of the resonator system falls off with increasing fre- 

 quency and pin penetration. 



Determination of the variation of frequency with position of the tuning 

 pins dictated the design of the drive mechanism to provide the necessary 

 range of motion. It was found that, with a pin diameter of 0.064 in. moving 

 in resonator holes of 0.088 in. diameter, the frequency band could be spanned 

 in a total travel of 0.110 in. Fig. 72 shows the nearly linear relationship 

 between frequency and pin penetration. 



The drive mechanism of the early Columbia models was redesigned to 

 make possible its fabrication by a single brazing operation in a jig and to 

 provide a sleev'e rather than a thread bearing. The backlash achieved in the 

 final design is such that one may reset the tuning head for a given frequency 



