60 BELL SYSTEM TECHNICAL JOURNAL 



lent network impedance, one outstanding feature is exemplified: 

 namely, the separation of the alternating- and direct-current com- 

 ponents. The equivalent networks are applicable to the alternating- 

 current fundamental component of the current and differ widely from 

 the direct-current characteristics. A complete realization of the im- 

 portance of this separation will be of advantage in the later steps 

 where extension of the classical theory to the case of ultra-high- 

 frequency currents is described. 



For a short time after the original introduction of the equivalent 

 network of the tube, affairs progressed smoothly. Soon, however, 

 frequencies were increased and a new complication arose. The diffi- 

 culty was attributable to the interelectrode capacities existing between 

 the various elements of the vacuum tube. The original attempts to 

 take this into account were based on the viewpoint that the tube 

 network should be complete in itself and separate from the external 

 circuit network to which it was attached. Correct results, of course, 

 were obtained by this method but later developments showed the 

 advantage of considering the equivalent network of the complete 

 circuit, including both tube and external impedances in a single net- 

 work. For instance, by grouping the combination of grid-cathode 

 capacity with whatever external impedance was connected between 

 these two electrodes, a great simplification occurred. This step also 

 has its analogy in the development of ultra-high-frequency relations. 



As time went on, higher and higher frequencies were desired, and 

 they were produced by the same kind of vacuum tubes operating in 

 the same kind of circuits, although refinements in circuit and tube 

 design allowed the technique to be improved to the point where 

 oscillations of the order of 70 to 80 megacycles were obtainable with 

 fair efficiency. When the frequency was increased still further, it was 

 found that extension of the same kind of refinements was unavailing 

 in maintaining the efficiency and mode of operation of the higher 

 frequency oscillations at the level which had previously been secured. 

 Ultimately, the three-electrode tube regenerative oscillator ceases to 

 function as a power generator in the neighborhood of 100 megacycles 

 for the more usual types of transmitting tubes. When this point was 

 reached, the external circuit had not yet shrunk up to zero proportions 

 and neither had its losses become sufficiently high to account altogether 

 for the failure of the tube to produce oscillations. From this point on, 

 the old-time cut-and-try methods were employed and marked im- 

 provements were secured. In fact, low power tubes have been made 

 which operate at wave-lengths of the order of 50 to 100 centimeters 

 with fair stability, although quite low efficiency. 



