___________________________________________________ Principles of Transmission Lines
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.
VOLTAGE CHANGE ALONG A TRANSMISSION LINE
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.
Transmission 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.
DC Applied to a Transmission Line
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.
3-19
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