MAGNETRON AS GENERATOR OF CENTIMETER WAVES 221 



but wliich have not come into general use will be mentioned in passing. One, 

 the so-called "serp)entine" anode structure, consists of a single slot, cut 

 into the anode body, which winds up and down the anode length and 

 around the interaction space. It is essentially a "half-section" wave guide, 

 closed on itself, oscillating in its fundamental at the cut-off wavelength. 

 As one passes along the resonator, the field for this mode is uniform, but, 

 by virtue of the geometrical arrangement, the field it supplies to the inter- 

 action space is t radians out of phase from gap to gap. Other modes corre- 

 spond to integral numbers of wavelengths along the length of the "serpen- 

 tine" resonator. The separation in frequency between the fundamental 

 and the next highest harmonic generally is not as great as is desirable. 



The other magnetron resonator system to be mentioned involves 

 the use of a single toroidal cavity of rectangular cross section whose inner 

 cyhndrical surface has been removed. Across this opening are placed the 

 anode segments, adjacent ones being connected to opposite sides. The 

 fundamental of this cavity corresponds to the cut-off wavelength as in the 

 "serpentine" structure. This cavity has been mentioned in the literature^" 

 and has received some attention during the war. It is most useful in low 

 voltage CW magnetrons where the small interaction space makes possible 

 a resonator with sufficiently great mode separation between the fundamental 

 and the first harmonic. 



7. Separation of Mode Frequencies 



7.1 Necessity and Means: The frequencies of the modes of the magnetron 

 resonator system near that of the tt mode would ordinarily be closely grouped 

 were not steps taken to separate them. Curve (a) of Fig. 25 shows the dis- 

 tribution of mode wavelengths for a typical 10 cm. resonator system like 

 that of Fig. 1. It is not easy to account for the exact nature of this curve. 

 By virtue of the fact that the mutual induction effects between circuits 

 bear different phase relations to the self induction effects for different modes 

 one would expect the mode frequencies to differ. However, the conditions 

 at the ends of the resonator block, in which region the flux lines link the 

 resonators, are extremely important and affect frequency in a way not com- 

 pletely explained as yet. That the end regions should have an effect is 

 understandable since they contribute capacitance and inductance as does 

 any other part of the resonating cavity in which there are electric and mag- 

 netic field hnes. Furthermore, as might be expected, it has been demon- 

 strated that the end conditions have a greater effect on mode frequency 

 the lower the mode number n. This results from the fact that the field 

 strength in a mode of periodicity n falls off inversely as rapidly as the nth 

 power of the distance from the resonator block. 



From the point of view of the electronics of the magnetron, one might 



^'This resonator is the so-called "turbator" discussed by F. Liidi, Bull. Schvveiz. 

 Elektrotechn. Verein, Vol. 33, No. 23 (1942). 



