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BELL SYSTEM TECHNICAL JOURNAL 



mine a maximum safe operating temperature. The anode material is 

 chosen to have a large coefficient of expansion and resistance to "creep" 

 at elevated temperatures. In choosing the form of the tuner it is necessary 

 to keep constantly in mind the necessity of maintaining the minimum ratio 

 of heat capacity to radiating surface. A minimum operating temperature 

 of the tuner is determined by heat flow to it from extraneous sources, l-'igure 

 84 shows the temperature as a function of bombardment power input for the 

 2K45 tuner. It will be observed that the heat from sources other than 

 bombardment produces a considerable rise in temperature. One principal 

 source of uncontrolled heating is radiation from the tuner cathode. This 



y \ 



2 5 3 3.5 4 4.5 

 ANODE POWER IN WATTS 



Fig. 84. — The tuner anode temperature as a function of the bombardment power. 

 The temperature rise at zero boml^ardment power is caused by radiation from the thermal 

 cathode and extraneous sources. 



is minimized by keeping the cathode as remote from the anode as is con- 

 sistent with the required electronic characteristics. 



When the maximum and minimum operating temperatures and the length 

 of the channel are determined the remaining problem is to determine and 

 offset for the tuning bow which will provide the optimum tuning characteris- 

 tics. We wish to obtain characteristics such that the heating time n and 

 the cooling time Tc are approximately equal and of a minimum value. The 

 choice of the bow offset also involves a choice of an initial gap spacing for the 

 resonator. On Fig. 83 the boundary values for the limiting temperatures Tq 

 and Tm are indicated by vertical lines. \\ ith any given bow offset which 

 corresponds to a particular tuning characteristic a limit is set on the initial 

 gap spacing by the requirement that the total motion of the bow between 



