1382 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



Fig. 8 shows the data for a tube of this type which has a cathode area 

 about six times that of the 313-C. This change in cathode area along 

 with the necessary change in anode geometry led to an impedance reduc- 

 tion of about five times. This type of design, of course, gives a large 

 increase in life of the tube but it is undesirable from the standpoint of 

 tube size. 



As pointed out above, low impedance can be attained by an increase 

 of the tube current and within limits by an increase of the cathode area. 

 A third parameter which may be varied is the density of the gas filling. 

 At a constant current and with a given cathode area, the resistive com- 

 ponent of the impedance decreases with increase in density or the pres- 

 sure, p, at a fixed temperature approximately in accordance with the 

 relation 





(4) 



The curve in Fig. 9 was obtained over a limited range of argon fillings 

 with a barium strontium oxide cathode. Since the current density in- 

 creases approximately in proportion to p^, the effective cathode area 

 will tend to be reduced unless the total current is kept high enough to 

 cover the cathode fully. Therefore, with a fixed total current there is a 

 limit to either increase in pressure or in cathode area. 



In further search for low impedance, a number of different gas fiUings 

 have been tried. They have included all the rare gases as well as several 

 mixtures of them. No significant advantages were obtained by the use 

 of other than the more common neon or argon gas fillings. 



We therefore see that the abihty to control impedance properties of 



60 



50 



O 



^ 40 



lUl 



20 



U « 

 ^ 



400 



800 1200 1600 2000 

 FREQUENCY IN CYCLES PER SECOND 



2400 



2800 



Fig. 8 — Resistive and inductive components of impedance for a large area 

 cathode tube as a function of frequency, /dc = 25 mm. 



