Stability of Negative Impedance Elements in Short Transmission Lines

For a period of about 40 years voice frequency repeaters have been engineered to provide amplification for both directions of conversation in two-wire telephone lines. Some of these repeaters have been operated in lines over 50 miles long; others have been operated in lines shorter than 10 miles. Yet practically all, including negative impedance repeaters of the E-type 1 , have been associated with transmission lines which can be classed as electrically long in that they have exceeded one half wavelength at the highest frequency in the pass-band. Within the past few years the need for two-way amplification in electrically short lines in the exchange area plant has become increasingly evident. A short section of line has limited phase shift at voice frequencies, and advantage can be taken of this fact in the repeater design, to reduce the over-all attenuation below that obtainable with design 333 334 TIIE B E L L SYSTEM T E C H N I C A L J O U R N A L , MARCH 1955 methods applicable to electrically long lines. Furthermore, the use of negative impedance devices such as E-type repeaters has made it possible to consider engineering the repeater as an integral part of an electrically short line. This method of design is a logical one because in addition to the reduction in over-all attenuation, the image impedances seen looking into the line terminals are modified by the addition of the repeater. In effect, the philosophy of the hybrid coil and the '22-type repeater is discarded along with the idea that the image impedance of the repeater must match the characteristic impedance of the line.