572 BELL SYSTEM TECHNICAL JOURNAL 



of the effective capacitance of the gap. As the capacitance increases in 

 tuning to lower frequencies the I R losses therefore increase. 



The objective of the coupling design may not be to obtain maximum 

 power output at all points in the band but rather to obtain uniformity of 

 electronic tuning and power output. An additional requirement in some 

 cases is that a minimum sink margin as defined in section Mil should be 

 maintained. This is equivalent to the problem arising in magnetron design 

 of controlling the pulling figure. 



In designing an appropriate wave guide coupling a number of variables 

 are at one's disposal. In the case of the W.E. 2K25 the variables available 

 are, the length of the tube probe exposed within the guide, the oflfset of the 

 probe from the axis of the guide, and the distance from the probe to the 

 shorting piston in the guide. In addition, the characteristics of the output 

 line of the tube are adjustable, and, finally, the coupling of the loop to the 

 resonator can be adjusted. As one might expect, there is a large number of 

 solutions with so many variables available. The most desirable solution 

 is one which provides a low standing wave ratio in all parts of the coupling 

 system. The method employed in the present case was to design a wave 

 guide to coaxial transducer which would provide a smooth broad band 

 transition from the wave guide characteristic admittance to the admittance 

 of the coaxial line. In the ideal case, the characteristic admittance of the 

 coaxial line to the loop should be maintained as uniform as possible. Struc- 

 tural considerations in the present case led to discontinuities which had to 

 be appropriately balanced in the final transducer. Dr. W. E. Kock of the 

 Bell Telephone Laboratories has given an expression which, for thin probes, 

 relates the probe length, the offset and the distance of the backing piston 

 when given the characteristic admittance of the coaxial line and the dimen- 

 sions of the guide between which a match is required. Once such a trans- 

 ducer has been obtained, the admittances which must be presented to it in 

 order to obtain maximum power from the oscillator are measured over the 

 band. From such measurements it is then possible to determine the cor- 

 rections in the loop size and in the transducer to obtain a given sink margin 

 throughout the band. This last step actually involves a certain amount of 

 cut and try in an effort to obtain satisfactory performance in all respects. 

 Figure 70 illustrates the transducer developed for the W.E. 2K25 oscillator 

 for use with \" by \" wave guide. All tests made on this tube are specified 

 in terms of operation in this coupler and with a load having the characteris- 

 tic impedance of the \" x \" guide presented to the coupler. 



Figure 71 shows a performance diagram for a typical W.E. 2K25 oscillator 

 operating in the coupler of P^ig. 70. The reference plane for the diagram is 



"J. B. Fisk, H. D, Hagstrum and P. L. Hartman, "The Magnetron as a Generator of 

 Centimeter Waves", B. S. T. J. Vol. XXV No. 2, pp. 167-348 (April, 1946). 



