___________________________________________________ Principles of Transmission Lines
is called the reflected wave. The resultant voltage curve (view B of figure
3-26) shows that the voltage is maximum at the end of the line, a condition
that occurs across an open circuit.
3-103. Another step in investigating the open-circuited RF line is to see how
the current waves act. The incident current wave is the solid line in figure
3-26, view C. The voltage is represented by the dotted line. The current is in
phase with the voltage while traveling toward the receiving end. At the end
of the line, the current is reflected in the opposite polarity; that is, it is
shifted 180 degrees in phase, but its amplitude remains the same. Dashed
lines show the reflected wave of current in view C. The heavy-line curve
represents the sum of the two instantaneous currents and is the resultant
wave. Notice that current is zero at the end of the line. This is reasonable,
because there can be no current flow through an open circuit.
3-104. Views B and C of figure 3-26 show the voltage and current
distribution along a transmission line at a point about one-eighth wavelength
after a maximum voltage or current reaches the end of the line. Because the
instantaneous values are continuously changing during the generation of a
complete cycle, a large number of these pictures are required to show the
many different relationships.
3-105. Figure 3-27 shows the incident and reflected waveshapes at several
different times. The diagrams in the left column of figure 3-27 (representing
voltage) show the incident wave and its reflection without change in polarity.
In figure 3-27, waveform (1), the incident wave and the reflected wave are
added algebraically to produce the resultant wave indicated by the heavy
line. In waveform (2), a zero point preceding the negative-going cycle of the
incident wave is at the end of the line. The reflected wave and incident wave
are 180 degrees out of phase at all points. (The reflected wave is the positive
cycle that just preceded the negative cycle now approaching the end of the
line.) The resultant of the incident and reflected waves is zero at all points
along the line. In waveform (3), the waves have moved 1/8 wavelength along
the line; the incident wave has moved 45 degrees to the right, and the
reflected wave has moved 45 degrees to the left. The resultant voltage, shown
by the heavy line, has a maximum negative at the end of the line and a
maximum positive 1/2 wavelength from the end of the line.
3-33
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