222 BELL SYSTEM TECHNICAL JOURNAL 



think proximity of mode frequencies to be no problem because the different 

 modes, even if of the same frequency, generally require different conditions 

 relating the operating parameters V and B for oscillation [see relation (14)]. 

 From the circuit point of view, however, close proximity of the mode fre- 

 quencies is clearly undesirable, for under such conditions it is possible that 

 a second mode may be excited by the electronically driven mode which is 

 usually the tt mode. The tt mode oscillation, coupled to the second mode 

 through some asymmetry in the resonator system, under these conditions 

 sets up forced oscillations in the second mode. The interaction field pattern 

 of the second mode then appears as a contamination of the tt mode pattern, 

 adversely affecting the electronic interaction with the tt mode. 



The mode frequency separation in magnetron resonator systems generally 

 has been accomplished by two methods which bear an instructive relation 

 to the means by which the mode frequencies of two coupled resonators may 

 be separated. In the two resonator system, it is clear that the mode fre 

 quency separation can be increased by increasing the coupling since the 

 difference in mode frequencies is brought about by the mutual coupling 

 between the circuits, as has been explained. On the other hand, mode 

 frequency separation may also be accomplished by detuning the individual 

 resonators relative to one another. In either case the mode frequencies 

 separate. Under shock excitation of the system, the beat frequency, equal 

 to half the difference in the two mode frequencies, increases, corresponding 

 to the greater rate of interchange of energy between one resonator and 

 the other. 



In the multicavity resonator system of the magnetron these means corre- 

 spond, on the one hand, to the increase of coupling by conductive connec- 

 tions between the resonators, or so-called straps, and, on the other hand, to 

 the use of cavities tuned alternately to different frequencies. This latter 

 method has been used in the so-called "rising sun" anode structure to be 

 discussed presently. 



7.2 Strapping of the Resonator System: The idea of strapping a magnetron 

 anode appeared in a British attempt to lock the oscillation of the resona- 

 tor system into the tt mode by connecting alternate anode segments together 

 with wire straps. Although the number of modes of such a strapped struc- 

 ture is not changed, since its N-fold symmetry remains, the so-called "mode- 

 locking straps" did succeed in separating the modes and making for easier 

 oscillation in the ir mode alone. The frequency separation of the modes is 

 not infinite, however, because the straps are not of negligible length com- 

 pared to a wavelength and thus have appreciable impedance between points 

 on the structure to which they are connected. In most magnetron resonator 

 systems today, straps of some form or other are employed. 



In Fig. 24 are shown four types of strapping including the early British 



