considerably greater than the amount that can be accounted for by the

impedance of the load in series with the resistance of the line.

3-40. 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. Because 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 in this chapter. 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. Because 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 is used throughout the following discussion of

transmission lines; but keep in mind that the principles presented here apply

to all transmission lines. We explain the theories using lumped constants and

distributed constants to simplify these principles further.

3-41. 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. Transmission line constants are distributed, as described below.