TC 9-64 _________________________________________________________________________
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.
LUMPED CONSTANTS
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.
3-10
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