MAGNETRON AS GENERATOR OF CENTIMETER WAVES 251 



cross sectional area of the straps. The effects of these changes upon the 

 mode frequencies may be seen from the considerations of the effect of straps 

 on the mode frequencies of the unstrapped resonator system already dis- 

 cussed. The increase in strap capacitance increases the wavelength of the 

 IT mode; the decrease in strap inductance decreases the wavelength of modes 

 of smaller n. As the tuning member is inserted, the mode frequencies 

 separate, and no limitation on the range of operation in the w mode is im- 

 posed by interference from other modes. This means has been used for 

 tuning ranges of better than ±6%, but there is nothing inherent in the 

 scheme to prevent its use for ranges considerably in excess of this value. 

 Tuning of the magnetron resonator system by any of the means described 



/c 



above alters its characteristic admittance, Yoc = A/ j , and hence the stored 



energy. For fixed output coupling and load admittance this amounts to 

 a variation of the effective loading as specified by the external Q. In some 

 cases, attempts have been made to compensate for this by designing into 

 the output circuit a frequency characteristic which keeps the external Q, 

 and hence the pulling figure, more nearly independent of frequency. 



Each of the tuning schemes described above is adaptable to precise and 

 specific frequency adjustment or to frequency variation at a slow rate. For 

 tuning over small ranges it is possible to vary the magnetron frequency 

 electronically, enabling one to frequency modulate its output at a high rate. 

 Of the schemes tried for this purpose there may be mentioned that in which 

 an intensity modulated electron beam of a specific velocity is shot parallel 

 to a superposed DC magnetic field through one of the magnetron resonators 

 or a closely coupled auxiliary cavity. 



Concerning the variation of magnetron operating frequency will be men- 

 tioned finally the shifts which are brought about by temperature variations 

 of the resonator block. Since the resonator system is generally constructed 

 entirely of copper, it expands or contracts uniformly with temperature and 

 the frequency shifts are those attained by a uniform scaling of the resonator 

 system by a very small factor. The temperature coefficient of frequency 

 may readily be seen to equal the negative of the linear coefficient of thermal 

 expansion of copper. 



10.4 Electronic Effects ou Frequency: In Section 3.6 Induction by the 

 Space Charge Cloud it has been seen that the electrons moving in the inter- 

 action space of the magnetron oscillator contribute an admittance Ye con- 

 nected to the resonator system. The magnitude of the negative electronic 

 conductance determines the energy delivered to the circuit and thus the 

 amplitude of oscillation. The electronic susceptance, arising from the phase 

 relation between the space charge spokes and the maximum of the tangential 



