The Limitation of the Gain of Two-Way Telephone Repeaters by Impedance Irregularities

ECAUSE of the fact that it is a difficult and expensive matter to build and maintain the high grade circuits that are required for modern long distance telephone transmission with repeaters, many workers in this field have attempted to devise some form of two-way repeater which would be able to give as large a gain as desired without singing or poor quality due to irregularities existing in the lines. They have thought that if such a repeater could be constructed it would permit the use of lines less carefully built and, therefore, cheaper than are at present required, and that fewer repeaters would be required because larger gains could be obtained at each repeater. As a matter of fact the irregularities in the lines have a very important effect and control, to a great extent, the repeater gains which can be used whenever a telephone circuit is arranged so as to be capable of transmitting in both directions over a single pair of wires with constant efficiency. It is the object of this paper to explain, in a very simple way, why this is true. To do this the phenomenon of electrical reflection is first made clear. Then a two-way repeater system is introduced and the effects of reflection upon this system are explained. After mentioning several of the types of repeaters which have been used successfully, the paper concludes with an explanation of the fallacies underlying a number of schemes which have been proposed from'time to time by various inventors. REFLECTION IN TELEPHONE LINES