356 



BELL SYSTEM TECHNICAL JOURNAL 



current balanced out and the remainder indicated by a 30-micro- 

 ampere meter, the sensitivity is more than sufficient to tune the circuits 

 correctly. 



The criterion of correct tune is the degree of suppression of standing 

 waves on the transmission lines. To determine whether or not the 

 maximizing adjustment insures an adequate standing wave sup- 

 pression, a standing wave detector was incorporated in the experi- 

 mental design. This is shown in Fig. 12. It consists of about 16 

 meters of 78-ohm coaxial line arranged in a coil and terminated by the 



Fig. 12 — The standing wave detector comprising 50 feet of 3/8-inch coaxial line, which 

 may be used to test the correctness of the input circuit adjustment. 



first circuit to be tested. It is fed at the other end by a test oscillator. 

 Six capacitively coupled taps are brought to the low-capacitance 

 switch shown in the photograph. The selector arm connects the taps 

 to an auxiliary receiver with a high-input impedance. The absence 

 of standing waves is shown by equal readings at the six positions. 

 It was found that the maximizing adjustment results in a standing 

 wave with less than ten per cent total variation, which represents 

 nearly as much suppression as the smoothness of the line allows. 



