3-18
impedance of the line. The line has properties other than resistance that affect input impedance. These
properties are inductance in series with the line, capacitance across the line, resistance leakage paths
across the line, and certain radiation losses.
Q22. What is the range of the characteristic impedance of lines used in actual practice?
VOLTAGE CHANGE ALONG A TRANSMISSION LINE
Let us summarize what we have just discussed. In an electric circuit, energy is stored in electric and
magnetic fields. These fields must be brought to the load to transmit that energy. At the load, energy
contained in the fields is converted to the desired form of energy.
Transmission of Energy
When the load is connected directly to the source of energy, or when the transmission line is short,
problems concerning current and voltage can be solved by applying Ohm’s law. When the transmission
line becomes long enough so the time difference between a change occurring at the generator and the
change appearing at the load becomes appreciable, analysis of the transmission line becomes important.
Dc Applied to a Transmission Line
In figure 3-18, a battery is connected through a relatively long two-wire transmission line to a load at
the far end of the line. At the instant the switch is closed, neither current nor voltage exists on the line.
When the switch is closed, point A becomes a positive potential, and point B becomes negative. These
points of difference in potential move down the line. However, as the initial points of potential leave
points A and B, they are followed by new points of difference in potential which the battery adds at A and
B. This is merely saying that the battery maintains a constant potential difference between points A and
B. A short time after the switch is closed, the initial points of difference in potential have reached points
A’ and B’; the wire sections from points A to A’ and points B to B’ are at the same potential as A and B,
respectively. The points of charge are represented by plus (+) and minus (-) signs along the wires. The
directions of the currents in the wires are represented by the arrowheads on the line, and the direction of
travel is indicated by an arrow below the line. Conventional lines of force represent the electric field that
exists between the opposite kinds of charge on the wire sections from A to A’ and B to B’. Crosses (tails
of arrows) indicate the magnetic field created by the electric field moving down the line. The moving
electric field and the accompanying magnetic field constitute an electromagnetic wave that is moving
from the generator (battery) toward the load. This wave travels at approximately the speed of light in free
space. The energy reaching the load is equal to that developed at the battery (assuming there are no losses
in the transmission line). If the load absorbs all of the energy, the current and voltage will be evenly
distributed along the line.
