252 BELL SYSTEM TECHNICAL JOURNAL 



retarding field, affects the frequency of oscillation. Under equilibrium 

 conditions the magnitude of the current, Irp , induced in the anode seg- 

 ments, its phase relative to the RF voltage between the segments, and the 

 operating frequency adjust themselves such that this admittance, Y , = 

 Irf/Vrf , equals the circuit admittance Y, . The induced RF current and 

 its phase relative to the RF voltage both depend upon the parameters such 

 as V and B, governing the electronic operation of the magnetron. The 

 frequency change at constant load arising from changes in F or 5 when 

 divided by the change in the DC current, I, drawn by the magnetron, is 

 called the frequency "pushing" and is measured in mc/s per ampere. 



A further effect of the electronic susceptance is the shift of the resonant 

 frequency between the oscillating and non-oscillating conditions of the 

 magnetron. In general the oscillating frequency is lower than the resonant 

 frequency of the non-oscillating magnetron. Thus the electronic susceptance 

 is capacitive with the space charge spokes moving somewhat ahead of the 

 field maxima during oscillation. This shift in the resonant frequency is 

 important in pulsed radar systems where the same antenna is used for both 

 receiving and transmitting. An echo of the transmitted pulse on its return 

 then encounters a high, oflf-resonance impedance at the magnetron which 

 absorbs very little of the returned energy. Most of the received pulse 

 energy is consequently made available to the receiver. For some magne- 

 trons the shift ofif resonance is not sufficient and other means such as the 

 use of the so-called ATR box are required to divert the received pulse energy 

 into the receiver. 



10.5 Frequency Spectrum of a Pulsed Magnetron: Only if a generator 

 operates for an infinitely long time is its output "monochromatic", that is 

 of a single frequency. The period of operation of a CW oscillator is generally 

 long enough to make any deviations from this unobservable. 



If, however, the oscillation is modulated as in the case of pulsed magne- 

 trons for which the pulse duration is of the order of one microsecond, it is 

 readily detectable that the output is "polychromatic" with the energy 

 distributed throughout a band of frequencies. The plot of the distribution 

 in frequency of the energy generated is called the frequency spectrum. 



This state of affairs is perhaps made plausible if one considers pulsing 

 the magnetron to be a very drastic means of ampUtude modulating its out- 

 put. Already, in connection with the case of two coupled circuits, it has 

 been seen how amplitude modulation of an oscillating system in time has 

 associated with it the distribution of the energy over more than one fre- 

 quency. In an analogous but more complicated manner, a sinusoidal 

 oscillation which is amplitude modulated by a nonsinusoidal pulse shape 

 like that of Fig. 37 (a) is compounded of frequencies not now discrete but 



