KEFLEX OSCILLA TORS 



623 



In considering resonators we should further note that the magnetic flux 

 must be produced by a flow of current, either convection current or dis- 

 placement current, around the lines of force. In a transformer this can be 

 identified as the current flowing in the coils. In the resonators used in 

 reflex oscillators it is the current flowing in the walls and, as displacement 

 current, across the gap and from one face of the resonator to the other. 



Two axially symmetrical resonators suitable for use in reflex oscillators 

 are shown schematically in Figs. 112 and 113. The resonator in Fig. 112 

 has grids and might be used with a broad unfocused electron beam at a low 

 d-c voltage; that shown in 113 has open apertures and might be used with a 



COUPLING 

 LOOP 



H 



COAXIAL LINE 



Fig. 112. — An oscillator cavity with grids and loop coupling to a coaxial line. 



IRIS WAVE-GUIDE 



Fig. 113. — An oscillator cavity without grids and with iris cou[)ling to a wave guide 



focused high voltage electron beam. In Fig. 112, the resonator is coupled 

 to a coaxial line by means of a coupling loop or coil; in Fig. 113 the resonator 

 is coupled to a wave guide by means of a small aperture or "iris." 



Let us consider the resonator of Fig. 112 in the light of what we have just 

 said. A magnetic field flows around the axis inside of the resonator. There 

 is an electric field between the top and bottom inside surfaces of the resona- 



at a depth. 



6 = 3.82 X W-'^^^X 



(al) 



Here X is wavelength in centimeters. It may also be convenient to note that the surface 

 resistivity of a centimeter square of resonator surface is, for copper, 



R = .045/Vx 



(a2) 



This means that if a current of I amperes flows on a surface over a width \V and a length 

 /, the power dissipation is 



F = r-iv./w 



(a3j 



For other non-magnetic metals, both 6 and R are ])r()p()rti()nal to the square root of the 

 resistivity with respect to that of copper. 



