MAGNETRON AS GENERATOR OF CENTIMETER WAVES 225 



given ring is connected. This determines the amount of current which the 

 strap carries. 



In the case of the tt mode the two straps are tt radians out of phase, each 

 strap being connected to points which are in phase and at potential maxima 

 [compare Figs. 23 and 24 (d)]. Their effect is predominantly capacitive. 

 The only currents flowing in the straps are the charging currents of the strap 

 capacitances. If a resonator system having a total capacitance C, a total 

 inductance L, and a v mode angular frequency coo , is strapped by a strapping 

 system which adds a total capacitance d to the resonator system, the new 

 frequency is 



oj^ = i/VL{c + Cs) = ojo/vTTcvc. 



The change in frequency is thus specified by the so-called "strength" or 

 "tightness" of the strapping implied in the ratio of strap to resonator 

 capacitance. 



For modes of lower periodicity, n < N/2, the average potential difference 

 between the straps and thus their capacitive effect is less because the straps 

 connect points on the resonator structure differing in phase by less than tt 

 radians. This corresponds to the shunting of a resonant line by a capaci- 

 tance nearer the voltage node, at which point it would have no effect. On the 

 other hand, a giieti ring now connects points on the anode whose potentials 

 differ in phase. The ring thus provides additional conducting paths for the 

 circulating RF currents in the resonator system. These paths are essentially 

 shunt inductances across the resonators which reduce the over-all inductance 

 of the resonator system, shifting the mode to shorter wavelength. As mode 

 number decreases the two straps come closer together in potential but each 

 strap connects points of greater potential difference. Thus the capacitive 

 effect decreases, the currents carried by the straps and hence the inductive 

 effect increase, resulting in a progressive depression of mode wavelength. 



In Fig. 25 the curves (a), (b), (c), and (d) represent the progression from 

 the unstrapped case, (a), through three successive cases of increasing 

 strength of strapping. This increase in strength of strapping has resulted 

 both in an increase of strap capacitance and a decrease of strap inductance 

 as the increase of tt mode wavelength and the decrease of mode wavelengths 

 for smaller n demonstrates. It is accomplished by increasing both the inter- 

 strap and strap-to-body capacitances as well as the cross sectional area of 

 the straps. 



The magnitude of the inductance of a strap depends on its physical length 

 between the points on the anode structure to which it is connected. As 

 this length increases, the strap inductance increases and hence has less effect 

 as a shunt path. For this reason, the effectiveness of a given strapping 

 scheme in producing separation of mode frequencies is reduced if the anode 



