REFLEX OSCILLATORS 597 



G. An Oscillator With Waveguide Out put— The 2K50 



Late in the war it became apparent that there was an urgent need for 

 radar systems which would permit a very high degree of resolution. Such 

 resolution requires the use of the shortest wavelengths possible, and as a 

 practical step development work was undertaken in the neighborhood of 

 1 cm. 



Work at these Laboratories led to a tube which produced over 20 milli- 

 watts and was thermally tunable over the desired frequency range by means 

 roughly similar to those employed in the 2K45. This tube had a wave guide 

 output. It had no grids; a sharply focused beam passed through a .015" 

 aperture in the resonator. The tube operated at a cavity voltage of 750 

 volts. 



Work by Dr. H. V. Neher at the M.LT. Radiation Laboratory resulted 

 in a design for an oscillator using grids which operated at a resonator voltage 

 of 300 volts. At the request of the Radiation Laboratory, the Bell Tele- 

 phone Laboratories undertook such development and modification as was 

 necessary to make the design conform to standard manufacturing techniques. 

 This work was carried out with the close cooperation of Dr. Neher. 



Figure 89 shows an external view of the tube and Fig. 90 a cross sectional 

 view of the final structure. There are two striking departures in this tube 

 from the designs previously described. One of these is that the axis of 

 symmetry is no longer parallel to the axis of the envelope but instead is 

 perpendicular to it. This construction makes possible in part the other 

 striking feature of the tube, which is the wave guide output. A number 

 of factors combine to make this type of output desirable and prac- 

 tical. The resonant cavity for a wavelength near L25 cm. becomes ex- 

 tremely small. Were loop coupling used this would necessitate a very small 

 coupling loop and also a very small diameter output line. The small dimen- 

 sions with loop coupling would require tolerances extremely difficult to 

 maintain with conventional vacuum tube techniques. On the other hand, 

 the wave guide used at L25 cm. is of dimensions (.170" x .420") such that 

 a wave guide output with a choke coupling can readily be incorporated in a 

 standard vacuum tube envelope. 



The wave guide coupling is accomplished by means of a tapered wave 

 guide which couples to the cavity through a non-resonant iris. The guide 

 tapers in the narrow dimension only, from the iris to a circular output 

 window. The tapered guide couples to the window by means of a circular 

 half wave choke. The VSWR introduced by the window is 1.1 or less. 

 External to the tube, there is an insulating fitting which permits the tube 

 to be coupled directly to the guide by means of a second choke coupling. 

 This makes it possible to operate the shell of the tube at a different potential 



