234 BELL SYSTEM TECHNICAL JOURNAL 



pedance, R, at the resonator presented by the output circuit must be small. 

 The characteristic impedance, Zo , of the waveguide is large. The trans- 

 former usually consists of a quarter wavelength section of characteristic 

 mpedance equal to the geometric mean of R and Zo , \^RZo ■ The vacuum 

 seal is made by a dielectric window in the waveguide. The quarter wave 

 transformer section may be a parallel plate transmission line cut in the 

 resonator block or may be a waveguide line of rectangular or dumb-bell 

 shaped cross section. Here again, the specific amount of its transformer 

 action must be adjusted, usually by variation of the small dimension, until 

 it provides the proper value of (Jext . 



Another type of output circuit involves coupling a coaxial line directly 

 onto the straps. This represents practically a pure case of the type of 

 coupling shown in Fig. 29 (c). 



8.2 Load: The load admittance which the output line presents to the 

 output circuit of the magnetron oscillator depends upon the characteristic 

 admittance, Fo , of the hne and upon the manner in which the line is termi- 

 nated. In discussing the single resonator of the magnetron resonator sys- 

 tem as a section of lossless transmission line terminated by a short circuit, 

 the input admittance and its relation to the standing waves on the line were 

 mentioned. Since the termination reflects all the energy incident upon it 

 in the shorted line, the voltage standing wave ratio, a, defined as the ratio 

 of the maximum voltage to the minimum voltage along the line, is infinite. 

 The input admittance of a shorted section of length f has been given in 

 equation (26). 



In the general case in which the line is terminated by an admittance, Yt , 

 not all the energy incident upon the termination is reflected, the standing 

 wave, whose position is determinated by the phase of Yt , has a finite value 

 of cr greater than unity, and the input admittance is given by the expression 



Yt + jYo tan ?^ 



F=Fo fj. (29) 



Fo +7^7- tan — 



If the voltage reflection coefficient, r, is defined as the ratio of the complex 

 voltage amplitudes of the reflected and incident waves, Ar and Aj , 



j<t> Ar 



the standing wave ratio may be written 



^ \Ai\+\Ar \ ^ 1 + P 

 " \Ar[\-\AR\ 1-p' 



I 



