330 BELL SYSTEM TECHNICAL JOURNAL 



impedance transformation is accomplished by a quarter wavelength section 

 opening directly into the output wave guide at one end and into the outside 

 wall of one resonator at the other (see Fig. 30 and discussion in PART I). 

 The small height of the resonator system from pole piece to pole piece makes 

 it necessary to use a loaded line, in this case of H-shape cross section. The 

 nature of the output circuit may be seen in the photograph of a cutaway 

 model of the 4J52 in Fig. 75. 



The transformation from the wave guide impedance of about 400 ohms 

 through the iris formed by the junction of the H-shape and rectangular 

 wave guides and through the X/4 section inserts a resistance of about 2 ohms 

 in series with the resonator to which the output circuit is connected. End 

 efifects make the desired transformer length differ slightly from X/4. The 

 length necessar}^- to give a pure resistance at the input end was deter- 

 mined by measurements in a 10 cm. model with which the distance from 

 the rectangular wave guide to the first voltage minimum in the H-section 

 could be measured. 



The vacuum seal in the wave guide output circuit was made at a circular 

 window sealed in a Kovar cup, mounted between choke couplings as seen 

 in Fig. 75. The diameter of the window and its thickness were chosen so 

 that the reflection coefficient of the window would be quite low over a broad 

 band of wavelengths near 3 cm. Insertion of a dielectric such as the glass 

 window into the wave guide line increases the capacitance per unit length 

 of the line. Compensation for this to bring the characteristic impedance 

 back to the normal value is done by increasing the inductance per unit 

 length over the same region. This may be accomplished by reduction of 

 the long dimension of the wave guide resulting in a nearly square cross 

 section. The circular opening, preferable for glass sealing, is a compromise, 

 the critical diameter being determined by experiment. The relatively large 

 size of the window makes it capable of withstanding RF voltage breakdown 

 even at very high power. 



Control of the output coupling is most readily effected by varying the 

 width of the slot in the H-section. The over-all transformation properties 

 of the H-section have been analyzed theoretically and the results confirmed 

 by measurements on output circuit models. By means of this analysis 

 it is possible to estimate fairly well the effect of changes in the slot width 

 upon the pulling figure. To hold the latter within production limits of 

 ± 2 mc/s, the slot must be held to ±0.001 in. 



The output circuit of the magnetron is formed by building up the as- 

 sembly from sections in what has been called a "sandwich" type construc- 

 tion. The resonator system and the central portion of the output H-section 

 are machined into one slab of copper. The "sides" of the H-section are 



