578 BELL SYSTEM TECHNICAL JOURNAL 



electronic tuning range. Examples of such systems are installations in high 

 altitude air craft, in which wide variations in temperature and pressure 

 may be expected. 



It was highly desirable to have the frequency control electrical. One 

 means of obtaining such a control is through motion of the resonator grids 

 produced by the thermal expansion of an element heated electrically. A 

 step in this direction was taken in the Sperry Gyroscope Company 2K21 

 oscillator in which the resonator was tuned by the thermal expansion of a 

 strut heated by passing a considerable current through it. 



At the Bell Telephone Laboratories the design for a thermally tuned 

 beating oscillator was based on a method which permitted the control of a 

 small current at a high voltage. In general, controlled high voltages are 

 easily available both from power supplies and from control circuits. Fur- 

 ther, it seemed desirable that the control of the heating should require no 

 power. These considerations suggested that the heating of the thermal 

 tuning element be accomplished by electron bombardment. Through the 

 use of a negative grid to regulate the bombardment, the tuning control 

 became a pure voltage adjustment. The bombardment method made it 

 possible to utilize configurations in the tuner which would have been less 

 practical if resistance heating had been employed. 



An early reflex oscillator incorporating these ideas was the Western 

 Electric 2K45 vacuum tube. Fig. 75 shows an external view of the tube 

 which, except for the output coaxial line, looks like a forshortened 6L6 

 vacuum tube. The plug-in feature of the earlier mechanically tuned os- 

 cillators was maintained in this oscillator, which was designed to couple to 

 the waveguide circuit through the same transducer developed for the 2K25. 



Figures 76 and 77 are cross-sectional views of the 2K45 made at right 

 angles to each other. The thermal tuning mechanism is contained in the 

 upper part of the structure. It is a bimetallic combination consisting of a 

 U shaped channel and a multi-leaf bow. The channel is formed of a material 

 with a large coefficient of expansion and a high resistance to slow permanent 

 deformation or creep at elevated temperatures. At the ends of this channel, 

 tabs bent down at right angles to the channel axis, provide rigid vertical 

 support for the channel without interfering with axial expansion. These 

 tabs are connected to the resonator, which in turn is supported by the 

 vacuum envelope as closely as possible in order to minimize the thermal 

 impedance of the path. This connection also serves to cool the channel 

 ends. The multi-leaf bow is welded to the channel at its end. The leaves 

 are made of a material having a low coefficient of expansion and, as they are 

 fastened to the channel at its ends, they remain cool and do not expand 

 appreciably as the channel is heated. The purpose of the multi-leaf con- 



