Linear Electrical Constants of a Transmission Line

Series Resistance R. The series resistance R is a "constant" that is found by dividing the total power losses occurring in the series portion of unit length of transmission line by the square of the current in that section. Thus, the series resistance losses are losses caused by the current flow and the accompanying magnetic field, including the direct-current resistance of the wire, and the energy losses, if any, caused by skin effect, eddy currents, and hysteresis. Series resistance R is usually expressed in ohms per loop mile for telegraph and telephone lines, and in ohms or microhms per loop meter for radio transmission lines. A loop mile consists of two miles of wire.

Series Inductance L. Series inductance L is the series self-inductance of the line wires. The current in a line wire causes an alternating magnetic field around the line wire, and this field induces an electromotive force in series in the line wire. This back electromotive force is directly proportional to the inductance. Because the currents in the two line wires are in opposite directions, the magnetic effects tend to cancel, and, if the two wires of a transmission line are close together, the self-inductance is low. Series self-inductance is usually expressed in henrys per loop mile for telegraph and telephone circuits, and in millihenrys or microhenrys per loop meter for radio transmission lines.

Shunt Conductance G. Shunt conduction G is a "constant" that is found by dividing the total power losses occurring in the parallel portion of unit length of line by the square of the voltage between the length of line (P = E²/R = E²G). Shunt conductance losses are losses that are caused by the voltage existing between line wires, These losses are somewhat involved and include losses due to leakage currents, dielectric hysteresis losses, etc. For telephone transmission lines, the shunt conductance, or leakage conductance, as it is sometimes called, is expressed in micromhos per mile of two-wire line. In radio transmission lines the shunt conductance is usually neglected.

Shunt Capacitance C. Shunt capacitance C exists between the two wires of a transmission circuit, and an alternating current will flow through this capacitance if an alternating voltage exists between the line wires. For telephone transmission lines the capacitance is usually expressed in microfarads per mile of two-wire line. For radio transmission lines, the unit is usually microfarads or micromicrofarads per meter of line.