3-10
However, the picture changes considerably when a long line is used. Since most transmission lines
are electrically long (because of the distance from transmitter to antenna), the properties of such lines
must be considered. Frequently, the voltage necessary to drive a current through a long line is
considerably greater than the amount that can be accounted for by the impedance of the load in series with
the resistance of the line.
TRANSMISSION LINE THEORY
The electrical characteristics of a two-wire transmission line depend primarily on the construction of
the line. The two-wire line acts like a long capacitor. The change of its capacitive reactance is noticeable
as the frequency applied to it is changed. Since the long conductors have a magnetic field about them
when electrical energy is being passed through them, they also exhibit the properties of inductance. The
values of inductance and capacitance presented depend on the various physical factors that we discussed
earlier. For example, the type of line used, the dielectric in the line, and the length of the line must be
considered. The effects of the inductive and capacitive reactances of the line depend on the frequency
applied. Since no dielectric is perfect, electrons manage to move from one conductor to the other through
the dielectric. Each type of two-wire transmission line also has a conductance value. This conductance
value represents the value of the current flow that may be expected through the insulation. If the line is
uniform (all values equal at each unit length), then one small section of the line may 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 transmission lines. We will
explain the theories using LUMPED CONSTANTS and DISTRIBUTED CONSTANTS to further
simplify these principles.
LUMPED CONSTANTS
A transmission line has the properties of inductance, capacitance, and resistance just as the more
conventional circuits have. Usually, however, the constants in conventional circuits are lumped into a
single device or component. For example, a coil of wire has the property of inductance. When a certain
amount of inductance is needed in a circuit, a coil of the proper dimensions is inserted. The inductance of
the circuit is lumped into the one component. Two metal plates separated by a small space, can be used to
supply the required capacitance for a circuit. In such a case, most of the capacitance of the circuit is
lumped into this one component. Similarly, a fixed resistor can be used to supply a certain value of circuit
resistance as a lumped sum. Ideally, a transmission line would also have its constants of inductance,
capacitance, and resistance lumped together, as shown in figure 3-9. Unfortunately, this is not the case.
Transmission line constants are distributed, as described below.
