248 BELL SYSTEM TECHNICAL JOURNAL 



frequency. Studies have been made of the transient conditions prevailing 

 near the beginning of a pulse when, in establishing the steady state, the suc- 

 cessive reflections are returning to the magnetron and, as a consequence, 

 its frequency is changing. Except in a limited region on the Rieke diagram 

 where operation is completely uncertain, it has been shown that the magne- 

 tron will settle down to operation at one frequency dictated by the phase 

 of the first reflection, even if oscillation at two frequencies by the previous 

 analysis is possible. Gaps in the tuning curve of a tunable magnetron 

 correspond to the periodic traversal of this uncertain region as the frequency 

 is varied and the load reflection coefficient moves around a constant p circle 

 on the Rieke diagram. 



10.3 Magnetron Tuning: To tune the magnetron oscillator it is necessary 

 to vary a susceptance somewhere in its circuit. It has already been ob- 

 served how variation of load admittance when reflected into the resonator 

 system as a susceptance change results in frequency pulling. Although for 

 other reasons this variation is usually limited by output circuit design to 

 the order of 0.1% of the operating frequency, it could be increased and used 

 in special instances as a means of tuning the magnetron. Similarly the 

 susceptance of a stabilizing cavity coupled into the resonator system may 

 be varied by tuning the cavity. This method in general enables one to 

 tune over a wider range than does variation of load susceptance since the 

 resonator system is usually more tightly coupled to the stabilizing cavity 

 than it is to the load. 



The largest tuning ranges have been attained, however, when it has been 

 arranged to vary one of the frequency determining parameters of the mag- 

 netron resonator itself. Schemes have been devised which alter primarily 

 either the inductance or the capacitance of the resonant cavities. \'aria- 

 tion of the inductance has been found more convenient at the shorter wave- 

 lengths and variation of the capacitance easier at longer wavelengths. 



Variation of the inductance may be accomplished by the insertion of a 

 conducting pin into each resonator where the RF magnetic lines of force 

 are concentrated. In a system of hole and slot type resonators it is arranged 

 to move the pins in and out along or near the axes of the holes. Such an 

 arrangement is shown schematically in Fig. 36 (a). As the pins are inserted 

 they reduce the volume available for the magnetic flux, thus reducing the 

 inductance and increasing the frequency. Tuning ranges as great as ±7% 

 of the mean frequency have been attained by this means. In spite of the 

 fact that the ratio of strap to resonator capacitance remams nearly con- 

 stant, the separation of mode frequencies, as might be expected, decreases 

 with increasing frequency because of the increase in strap inductance re- 

 sulting from increase of its electrical length. The effect is not so large, 



