Figure 3-9.Equivalent circuit of a two-wire transmission line.
Transmission line constants, called distributed constants, are spread along the entire length of the
transmission line and cannot be distinguished separately. The amount of inductance, capacitance, and
resistance depends on the length of the line, the size of the conducting wires, the spacing between the
wires, and the dielectric (air or insulating medium) between the wires. The following paragraphs will be
useful to you as you study distributed constants on a transmission line.
Inductance of a Transmission Line
When current flows through a wire, magnetic lines of force are set up around the wire. As the current
increases and decreases in amplitude, the field around the wire expands and collapses accordingly. The
energy produced by the magnetic lines of force collapsing back into the wire tends to keep the current
flowing in the same direction. This represents a certain amount of inductance, which is expressed in
microhenrys per unit length. Figure 3-10 illustrates the inductance and magnetic fields of a transmission
Figure 3-10.Distributed inductance