REFLEX OSCILLATORS 593 



second transit of the gap. This tends to ehminate electronic tuning hystere- 

 sis and the repeller characteristic of the 2K45 is essentially free of this 

 phenomenon. This gun design has the further advantage that it avoids the 

 necessity for the first grid used in the 2K25. This eliminates the current 

 interception on this grid with a resulting increase in the effective current 

 crossing the gap. This type of gun also permits the design of a more efficient 

 resonator by reducing the grid losses. 



A second variation in design from the 2K25 is that in the 2K45 the second 

 grid moves with reference to the repeller. This has the advantage of reduc- 

 ing the variation of the repeller voltage for optimum power with resonator 

 tuning. The drift angle in a uniform repeller field is given by 



e-^Jf (13.3) 



where /is the spacing between the repeller and the second grid of the resona- 

 tor. If the same drift angle 6 is maintained at all frequencies in the band, 

 then the repeller voltage must vary. If ( is fixed as/ increases V r must also 

 increase in order to maintain a fixed fraction. If /varies and increases as / 

 decreases then a smaller variation in V r is required and in the particular 

 case that Ovaries inversely with/ the repeller variation may be made zero. 

 Usually other requirements determine the variation of /and it is not always 

 possible to make the variation zero. In the case of the 2K45 the variation 

 over the band is approximately half the amount which would occur if /were 

 fixed. 



The output coupling and fine of the 2K45 were designed so that the 

 oscillator would provide the desired characteristics in the same waveguide 

 adapter as designed for the 2K25. The power output as a function of 

 frequency for a typical tube is shown in Fig. 86a. Curves A, B and C of 

 Fig. 86 show the variation of power output with cavity tuning when the 

 repeller voltage is set for an optimum at the indicated frequency and held 

 fixed as the cavity tuning is varied. The frequency shift between half power 

 points in this case is very much wider than with repeller tuning. This is a 

 consequence of the fact that whereas in repeller tuning both the frequency 

 and the drift time change in a direction to shift the transit angle away from 

 the value for maximum power, with cavity tuning only the frequency 

 changes. Moreover, the fact that the repeller to second grid spacing in this 

 design varies with frequency tends to reduce the variation of the drift angle 

 with frequency. The envelope of the curves .4, B and C gives the power 

 output as a function of frequency when the repeller voltage is adjusted to 

 an optimum at each frequency. 



Fig. 86b gives the half power electronic tuning as a function of frequency 

 measured statically and also dynamically with a 60 cycle repeller sweep. 

 The difiference arises from thermal effects. 



