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BELL SYSTEM TECHNICAL JOURNAL 



to })r()vide as nearly as possible a uniform field in the region into which the 

 beam penetrates. 



A wide variety of cavity resonators has been designed for use with this 

 oscillator. An oscillator of this construction is fundamentally capable of 

 oscillating over a much wider frequency range than tubes tunable by means 

 of capacitance variation. The advantage arises from the fact that the inter- 

 action gap where the electron stream is modulated by the radio frequency 

 field is fixed. As discussed in more detail in Section X, this results in a 

 slower variation of the modulation coeflficient with frequency and also a 

 slower variation of cavity losses and gap impedance than in an oscillator in 

 w'hich tuning is accomi)lished by changing the gap spacing. A cavity 

 designed for wide range frequency coverage using the 707A tube is shown in 

 Fig. 60. Using such a cavity it is possible to cover a frequency range from 

 1150 to 3750 megacycles. The inductance of the circuit is varied by moving 

 the shorting piston in the coaxial line. For narrow frequency ranges, 



Fig. 60. — Sketch showing a piston tuned circuit for the VV.E. 707-A which will permit 

 operation from 1150 to 3750 mc. 



cavities of the type shown in Fig. 61 are more suitable. In such cavities 

 tuning is effected by means of plugs which screw into the cavity to change 

 its effective inductance. Power may be extracted from the cavity by means 

 of an adjustable coupling loop as shown in Fig. 61. 



The 707A was the first reflex oscillator designed to operate at a low voltage 

 i.e. 300 volts. This low operating voltage proved to be a considerable 

 advantage in radar receivers because power supplies in this voltage range 

 provided for the i.f. amplifiers could be used for the beating oscillator as 

 well. Operation at this voltage was achieved by using an interaction gap 

 with fine grids, which limits the penetration of high frequency fields. This 

 results in a shorter effective transit angle across the gap for a given gap 

 spacing and a given gap voltage than for a gap with coarse or no grids. 

 Hence, for a given gap spacing a gcod modulation coeflficient can be ob- 

 tained at a lower voltage. Moreover, since drift action results in more etii- 

 cient bunching at low voltages, a larger electronic admittance is obtained 

 than with an open gap. This gain in admittance more than outweighs the 



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