Passive Four-Pole Admittances of Microwave Triodes

T~"URING the course of an experimental study of the active four-pole admittances 1 of the 1553 close-spaced triode,2 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 admittances 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.