220 BELL SYSTEM TECHNICAL JOURNAL 



the asymmetry discussed above, one of the modes is strongly coupled and 

 the other weakly coupled. The strongly coupled mode is that for which 

 the anode potential wave is sine-like with respect to the cavity containing 

 the output coupling loop as origin. In this mode the current and hence the 

 magnetic flux in the output cavity is maximum. The fact that one of the 

 modes of periodicity n is weakly coupled maybe of significance in magnetron 

 operation. As will be discussed later, oscillation in a loosely coupled mode, 

 having a high Q by virtue of its being damped only by losses in the resonator 

 system itself, may build up more rapidly under electron drive than that of 

 the TT mode. Then it is possible for the magnetron to oscillate either steadily 

 under certain conditions, or intermittently, in an unwanted mode. For this 

 reason it is usually necessary to provide a second asymmetry in the anode 

 structure so as to shift the standing wave patterns of the two modes of same 

 n most likely to offend and in this way to equalize their coupling to the out- 

 put circuit. 



6.4 Higher Order Modes: To this point in the discussion the RF cir- 

 cuit of the magnetron has been assumed to behave like a string of N lumped 

 circuits coupled together in a ring, each circuit having only one natural 

 frequency of resonance. This structure, as has been seen, has N modes of 

 oscillation. Actually the multicavity resonator, since its constants are 

 distributed, has an infinite number of modes of oscillation. They are to 

 be distinguished by the nature of the variation of the RF field along the 

 axis of the resonator system and radially in the individual resonators. Thus 

 there may be nodal planes passing through the resonator system normal to 

 its axis, or nodal cyhnders, concentric with the axis of the system, passing 

 through the resonators. The modes may be classified as symmetric or 

 antisymmetric depending on whether the two ends of the system are in 

 phase or tt radians out of phase. The variation of RF voltage along the 

 anode length is a circular sine or cosine function if the mode frequency is 

 greater than the resonant frequency of the unstrapped resonator system 

 and is a hyperbolic sine or cosine function if the mode frequency is less. 

 The fundamental multiplet of N modes discussed above are symmetric 

 modes corresponding either to no variation or to a hyperboUc cosine varia- 

 tion of RF voltage along the length. In these modes of the resonator system 

 the cavities, considered as radial shorted transmission lines, resonate in 

 their fundamental modes. Generally the frequencies of the higher order 

 modes of the resonator system are quite far removed from those of the 

 fundamental multiplet and only rarely need be considered. 



6.5 Other Types of Resonators: As alluded to earlier, other types of 

 magnetron resonators have been devised which can supply the proper al- 

 ternate TT mode potentials to the segments of a multisection anode. Two 

 of these which have received some consideration by magnetron designers 



