602 



To Display Unit 



Fig. 11-28 Simplified Block Diagram of a Radar with a High-Gain Amplifier as 

 the Power Output Tube of the Transmitter. 



11-6 BACKWARD WAVE OSCILLATORS — CARCINOTRONS 



Another type of microwave tube that may ultimately find application 

 in airborne radar is the backward wave oscillator, or Carcinotron. These 

 oscillators employ a slow wave circuit that supports an unusual type of 

 electromagnetic wave in which the phase velocity is in one direction along 

 the structure and the group velocity is in the opposite direction. It can 

 be shown that all periodic, slow wave structures support these backward 

 waves. If an electron beam is directed along the structure, oscillations are 

 possible at a frequency at which the phase velocity of the backward wave 

 is synchronous with the velocity of the electron beam. The phase velocity 

 changes with frequency, and the significant feature of these oscillators is 

 that the frequency of oscillation depends upon the velocity of the electron 

 beam. The oscillator can be tuned over a wide range of frequencies by 

 varying the voltage of the electron beam. 



There are two types of backward wave oscillators. The first, the axial- 

 field type, resembles a traveling wave tube in that the magnetic field is 

 parallel to the electron beam, which passes along the axis of the slow wave 

 structure. This is illustrated in Fig. 11-29. This type of oscillator is also 

 known as a Type-0 Carcinotron. It is generally a low-power, low-efficiency 

 tube suitable principally for receiver and test-equipment applications. 



The second type of backward wave oscillator is the crossedfield type, 

 which resembles a magnetron in that the electric and magnetic fields are 

 perpendicular to each other rather than parallel. It is illustrated in Fig. 

 11-30. The electron trajectories are also perpendicular to the magnetic 

 field. The crossed-field backward wave oscillator is also called a Type-M 

 Carcinotron. The efficiency of this type is relatively high, and this high 



