the line m ay represent several feet. This illustration of a two-wire transmission line will be used
throughout the discussion of transmission lines; but, keep in mind that the principles presented
apply to all transm ission lines.W e will explain the theories using LUMPED CONSTANTS and
DISTRIBUTED CONSTANTS to further simplify these principle.
A transmission line has the properties of inductance, capacitance, and resistance just as
the m ore conventional circuits have. Usually, however, the constants in conventional c irc uits
are lum ped into a single device or com ponent. For exam ple, a coil of wire has the property of
inductance. W hen a certain am ount of inductance is needed in a circuit, a coil of the proper
dimensions is inserted.
The inductance of the circuit is lum ped into the one com ponent. Two m etal plates
separated by a small s pace, can be used to supply the required capacitance for a circuit. In
such a case, most of the capacitance of the circuit is lum ped into this one component. Similarly,
a fixed resistor can be used to supply a certain value of circuit resistance as a lum ped sum.
Ideally, a transm ission line would also have its constants of inductance, capacitance, and
resistance lumped together, as shown in figure 3-1. Unfortunately, this is not the
case.Transmission line constants are as described in the following paragraphs.
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.