THE RADAR RECEIVER 723 



plate-grid cavity and thence to the output coaxial Hne. The frequency 

 stabiUty of this particular design has been quite satisfactory due to the con- 

 siderable development effort expended on the mechanical design features of 

 this assembly. 



Another radar receiver beating oscillator which has been extensively 

 employed in military equipment designs operating at 3000 mc and higher 

 during the past war period is the reflex velocity-modulated oscillator. The 

 2K25 type, which is a typical single-cavity reflex oscillator for operation in 

 the 10,000-mc region, is illustrated in Fig. 17. The general principle of 

 operation is quite straightforward, but the complete theory of operation is 

 exceedingly complex and has been described in detail elsewhere.^" A beam 

 of electrons of relatively uniform velocity and density is projected from a 

 cathode surface toward a cavity space defined in part by two grids and then 

 toward a repeller electrode beyond. The presence of the oscillatory poten- 

 tial between the two cavity grids acts to impart initial velocities to the 

 electron stream in accordance with the cavity radio-frequency potentials 

 existing at the time of crossing of this gap. Under certain operating condi- 

 tions between these grids and the repeller electrode, which is maintained 

 at a negative potential, "bunching" of electrons occur and upon the return 

 of the electrons to the cavity region under the retarding influence of the 

 repeller electrode, a certain amount of power may be extracted and utilized 

 externally. As might be expected from this cycle of events, the optimum 

 operating conditions necessary for reinforcement of oscillation within the 

 cavity are related to the time required to return the electrons to the cavity 

 with reference to the instantaneous oscillatory radio-frequency potential of 

 the cavity. Thus, numerous modes of oscillation are found in this type of 

 reflex velocity-modulated oscillator which are related to each other by 

 integral numbers of periods of the osciUatory frequency and the transit time 

 of the electrons. In the practical application of the reflex oscillator the 

 number of useful modes are limited to perhaps two or three, the external 

 power output available at the additional theoretical modes being reduced 

 by dissipative conditions within the oscillatory system. The relation of 

 repeller potential to the appearance of these modes is illustrated in Fig. 18 

 for the case of a 2K25-type oscillator. It should be observed here that the 

 frequency at the maximum power output condition for each mode is the 

 same, i.e. the frequency associated with the cavity dimensions for that series 

 of modes. The cavity dimensions of the reflex oscillator tube are varied 

 by the application of external mechanical pressure regulated by an adjustable 

 tuning strut as shown in Fig. 17. The power for external use is obtained 

 from a coupling loop located withiji the cavity region and transmitted 

 through the base by means of a coaxial lead. 



" Pierce and Shepherd, Loc. Cit. 



