MAGNETRON AS GENERATOR OF CENTIMETER WAVES 311 



ducible output circuit for the 725A. The requirement of interchangeability 

 with the 2J21 restricted the output wave guide flange in its position relative 

 to the axis of the anode and the mounting flange. Previous 3 cm. magne- 

 trons having hole and slot resonators utilized coaxial output circuits coupled 

 to the resonator system by a loop located in the median plane of one of the 

 resonators as in Fig. 1. Although it was recognized that a wave guide 

 output for a small magnetron of this type would be more elegant and repro- 

 ducible than a coaxial output, diflliculty in twisting the plane of polarization 

 from the resonator to the output wave guide flange, involved in the inter- 

 changeability requirement, and the lack of experience with such outputs 

 made necessary the choice of an output for the final design similar to the 

 original. To obviate the removal of a large portion of one resonator to 

 accommodate the output and to make the loop easily accessible for inspection 

 and adjustment, it was placed in the end space directly over the circular hole 

 of one of the resonators. This so-called "halo" type loop is shown in Fig. 66. 

 The coaxial line was terminated, as in previous 3 cm. magnetrons, in a junc- 

 tion to wave guide, fabricated as part of the magnetron. The output circuit 

 was designed around a matched junction which, it was hoped, would have 

 the best electrical breakdown strength at the reduced pressures of high 

 altitude operation, would be frequency insensitive, and would avoid adjust- 

 ment of the junction on individual magnetrons. The entire antenna of the 

 junction was enclosed in the vacuum envelope of the magnetron as may be 

 seen in Fig. 66. 



The method of attack on the output circuit problem from this point on 

 may be summarized as follows: First, a matched junction from coaxial to 

 wave guide such that it could be applied to the final over-all design, had to be 

 achieved. Then, for a given choice of loop size and position relative to the 

 resonator, it was necessar}^ to obtain the transformer properties of the coaxial 

 line immediately adjacent to the loop so that the proper over-all coupling 

 from magnetron resonator to wave guide load was achieved. Here again 

 the mechanical restriction on the distance between the resonator and wave 

 guide axes was found to be cumbersome. Two loop sizes were tried, that 

 requiring the higher standing wave in the coaxial transformer to couple it 

 properly being later discarded. At any given stage in the development, the 

 procedure consisted of finding the impedance to be presented to the loop 

 by the coaxial output. This was done by transforming through the entire 

 output circuit to the loop terminals, the impedance needed in the output 

 wave guide for proper loading, using the transformer parameters measured 

 on a simulated output circuit. This impedance at the loop terminals was 

 obtained by variation of the transformer properties of the coaxial Una 

 between the loop terminals and the coaxial to wave guide junction. The 



