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BELL SYSTEM TECHNICAL JOURNAL 



for a tuner. There is a certain amount of arbitrariness in choosing the 

 limiting dimensions of the tuner. If the expanding element is to be short 

 it is necessary either to operate over a very wide range of temperatures or 

 else use some mechanical means of amplifying the motion obtained over a 

 more limited range. Since the more limited is the required temperature 

 range the greater is the tuning speed, it is obviously advantageous to use 

 mechanical amplification. As previously pointed out the electron bombard- 

 ment method of heating and radiation cooling are especially suitable to such 

 a mechanism because of the freedom of design they permit. Previous dis- 



TEMPERATURE 



Fig. 82. — The ideal type of deflection vs temperature characteristic desired for a ther- 

 mally tuned oscillator. The motion 6 is that required to shift the resonant frequenc\- 

 of the cavity through its required band. 



cussion has shown that the temperatures corresponding to zero and ma.ximum 

 power input must be separated by wide margins from the temperatures 

 corresponding to the limits of the tuning range. Any motion which occurs 

 in these margins is unnecessary and in general undesirable. Ideally, the 

 tuning mechanism should have a characteristic as shown in Fig. 82. The 

 type of tuning mechanism chosen for the 2K45 is a first appro.ximation to 

 such a characteristic, as is shown in Fig. 83, which gives a family of charac- 

 teristics corresponding to various initial offsets of the bow for a given length 

 of bow. The bows are formed to a sinusoidal shape. This structure gives 



