CHAPTER XV 

 MAGNETRON AMPLIFIER 



Synopsis of Chapter 



^TpHE HIGH EFFICIENCY of the magnetron oscillator is attributed to 



-■■ motion of the electrons toward the anode (toward a region of higher 

 d-c potential) at high r-f levels. Thus, an electron's loss of energy to the r-f 

 field is made up, not by a slowing-down of its motion in the direction of wave 

 propagation, but by abstraction of energy from the d-c field. ^ 



Warnecke and Guenard^ have published pictures of magnetron amphfiers 

 and Brossart and Doehler have discussed the theory of such devices.^ 



No attempt will be made here to analyze the large-signal behavior of a 

 magnetron amplifier or even to treat the small-signal theory extensively. 

 However, as the device is very closely related to conventional traveling- 

 wave tubes, it seems of some interest to illustrate its operation by a simple 

 small-signal analysis. 



The case analyzed is indicated in Fig. 15.1. A narrow beam of electrons 

 flows in the -\-z direction, constituting a current /o . There is a magnetic 

 field of strength B normal to the plane of the paper (in the x direction), and 

 a d-c electric field in the y direction. The beam flows near to a circuit which 

 propagates a slow wave. Fig. 15.3, which shows a finned structure opposed 

 to a conducting plane and held positive with respect to it, gives an idea of 

 a physical realization of such a device. The electron stream could come from 

 a cathode held at some potential intermediate between that of the finned 

 structure and that of the plane. In any event, in the analysis the electrons 

 are assumed to have such an initial d-c velocity and direction as to make 

 them travel in a straight line, the magnetic and electric forces just cancelling. 



The circuit equation developed in Chapter XIII in connection with trans- 

 verse motions of electrons is used. Together with an appropriate ballistical 

 equation, this leads to a fifth degree equation for F, 



1 For an understanding of the high-level behavior of magnetrons the reader is referred to: 



J. B. Fisk, H. D. Hagstrum and P. L. Hartman, "The Magnetron as a Generator of 

 Centimeter Waves," Bell System Technical Journal, Vol. XXV, April 1946. 



"Microwave Magnetrons" edited by George B. Collins, McGraw-Hill, 1948. 



^ R. Warnecke and P. Guenard, "Sur L'Aide Que Peuvent Apporter en Television Quel- 

 ques Recentes Conceptions Concernant Les Tubes Electroniques Pour Ultra-Hautes 

 Frequences," Annales de Radioelectricite, Vol. Ill, pp. 259-280, October 1948. 



^ J. Brossart and O. Doehler, "Sur les Proprietes des Tubes a Champ Magnetique Con- 

 stant: les Tubes a Propagation D'Onde a Champ Magnetique," Annales de Radioelectricite, 

 Vol. Ill, pp. 328-338, October 1948. 



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