Passive Four-Pole Admittances of Microwave Triodes 



By SLOAN D. ROBERTSON 



Measurements have been made of the passive, four-pole admittances of parallel- 

 plane triodes over a wide range of calhode-to-grid and grid-to-piate spacings at 

 a frequency of 4060 megacycles. Results are given for a parallel wire grid and a 

 cross-lateral grid. The microwave transadmittances are found to be much higher 

 than the values measured at low frc.iuencies. 



DURING the course of an experimental study of the active four-pole 

 admittances^ of the 1553 close-spaced triode,'- a question arose as to 

 whether the grid wires were introducing any appreciable inductance or 

 resistance in the circuit used for measurement. It appeared necessary, 

 therefore, to learn something of the passive four-pole parameters of the 

 triode in order to separate the electronic from the passive admittances. It 

 was generally believed that the electrostatic analyses of the passive admit- 

 tances which have been successfully applied at the lower frequencies would 

 no longer be valid with close-spaced structures at microwave frequencies. 

 For example, it was considered possible that the grid wires themselves might 

 possess an effective inductive reactance, so that the admittances between 

 the grid and cathode or between the grid and anode might not be equal to 

 the values computed from the electrostatic capacitances. Moreover, it was 

 thought likely that energy might be transmitted from the cathode-grid 

 region to the cathode-plate region or vice versa, not only by the medium of 

 the electrostatic coupling, but also by means of an electromagnetic coupling 

 through the grid. The measurements to be reported below indicate that the 

 first of these conjectures was false, but that the second was true. 



In view of the lack of available information on these questions in general, 

 it seemed highly desirable to employ the available measuring equipment, 

 not only to determine the passive parameters of a triode having electrode 

 spacings corresponding with those of the 1553, but to extend the scope of 

 the measurements to include a wide range of electrode spacings in order 

 that the results would be of more general interest. 



Although these measurements were in principle very simple, in practice 

 the mechanical problem of achieving the desired degree of accuracy proved 

 rather difficult. It was required that the cathode, grid, and anode planes be 

 almost perfectly parallel and that the spacuigs between them be adjustable 



^ S. D. Robertson, "Electronic Admittances of Parallel-Plane Electron Tubes at 4000 

 Megacycles," this issue of the B. S. T. J. 



^ J. A. Morton, "A Microwave Triode for Radio Relav," Bell Laboratories Record, 

 Vol. XXVII, No. 5, pp. 166-170, May 1^49. 



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