3-59. When a transmission line is "short" compared to the length of the RF

waves it carries, the opposition presented to the input terminals is

determined primarily by the load impedance. A small amount of power is

dissipated in overcoming the resistance of the line. However, when the line is

"long" and the load is an incorrect impedance, the voltages necessary to drive

a given amount of current through the line cannot be accounted for by

considering just the impedance of the load in series with the 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.

3-60. 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.

3-61. 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.

3-62. 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.

Integrated Publishing, Inc.
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