Type D-1 single channel short-distance carrier telephone system

For short distances of 150-200 miles multi-channel carrier systems are not, in general, economical, as their adjustment and maintenance is relatively complicated. The D-1 system described is much simpler, and has been designed to provide an additional circuit over distance as low as 50 to 75 miles at annual charges less than those of additional copper circuits. The system employs equipment which requires little adjustment, and operates with hardly more attention than an ordinary repeater. It can be used in stations which do not have regular maintenance, but are observed once or twice a day. The circuits have a high degree of stability and adjustments of gain, battery voltage and frequency are not required since these remain sensibly constant after the circuit is put into operation. The transmission characteristics are such that with one such set on a pole line, an overall equivalent of 3.5 db is possible if the attenuation is not over 20 db at the highest carrier frequency. Where several sets are on the same pole, the line attenuation must not exceed 14 db. These values are based on a transmitting gain of 6.5 db and a receiving gain of 10 db. No intermediate repeater is used. The carrier frequency for transmission is 10.3 kilocycles in one and 6.87 kilocycles in the other direction. The carrier is suppressed, and the lower side band transmitted. The D-1 system has proved very useful under emergency conditions. The paper describes the equipment and the circuits employed.