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BELL SYSTEM TECHNICAL JOURNAL 



wave intervals along the guide, properly distributed in impedance level and 

 all tuned close to the center frequency of the channel to be extracted, Fig 7(b). 

 The practical question was to find how to obtain these resonances in an easily 

 constructed and adequately adjustable form. 



Early experiments indicated that a probe inserted in the broad side of the 

 guide far enough so that its end formed an appreciable capacitance with the 

 opposite side could be made to resonate in a series resonant fashion. Imped- 

 ance levels available through this means of coupling were, however, far lower 

 than required. Accordingly an alternate method in which the probe is 



Fig. 6 — Hybrid junction. 



inserted in the narrow side of the guide was selected (Fig. 8). The probe is 

 made long enough to approach the opposite narrow wall of the guide and is 

 tipped by a capacitative disk. The capacity of the disk and consequently the 

 resonant frequency of the circuit is adjusted by means of a screw in the wall 

 just opposite the disk. Since this rod is inserted perpendicular to the narrow 

 guide wall it is normally uncoupled to the principal mode in the guide. An 

 adjustable coupling is achieved by inserting a screw in the broad side of the 

 guide just above the probe. Insertion of this screw disturbs the symmetry of 

 the field and couples the rod to the guide. Increasing the screw insertion 

 increases the coupling and consequently varies the impedance level of the 



