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BELL SYSTEM TECHNICAL JOURNAL 



circuit, tuned to the desired frequency, across the two anodes, as 

 shown in Fig. 28. Each anode deHvers energy to the oscillating 

 circuit during alternate half-cycles, so that in effect, it is equivalent 

 to a- push-pull oscillator. The limiting frequency as set by the inter- 

 electrode capacitances and lead inductances (corresponding to the 

 similar limit for the negative grid tube) is 450 megacycles. The de- 



X 



Fig. 28- 



-Oscillation circuit of split-plate magnetron, 

 this type of circuit. 



Data for Fig. 27 taken in 



crease in output before this limit is reached is due to resistance and 

 radiation losses and to the effect of electron transit time. 



The magnetron, as contrasted with the negative grid tube, will 

 oscillate with circuits having a high decrement. However, for its 

 most efficient operation the effective anti-resonant impedance of the 

 tuned circuit when loaded must be approximately 10 times the value 

 required by a triode with the same anode dimensions. The load re- 

 sistance that can be obtained at high frequencies is only a fraction of 

 this value, so that the efficiency becomes increasingly less with higher 

 frequencies. A further limitation is due to the fact that the electron 

 current is concentrated on only a small part of the anode surface. 

 This reduces the safe anode dissipation unless the anode is designed to 

 have a high thermal conductivity. Because of these limitations, the 

 ratio of output to the inter-electrode capacitance may be only slightly 

 more favorable than the corresponding ratio for a triode of the same 

 anode dimensions. 



Type Depending on Electron Transit Time 

 When the magnetic field of a split-plate magnetron is adjusted to 

 near the critical value given by equation (3), oscillations can be pro- 

 duced whose frequency will depend primarily upon the time of flight 

 of electrons between filament and anode in a way closely resembling 

 the behavior of the positive grid oscillator in its lower frequency mode 

 of oscillation. For best output, the field must be above the critical 



