3-33
instantaneous peak amplitude that is equal to the sum of the peak amplitudes of the incident and reflected
waves. Since most indicating instruments are unable to separate these voltages, they show the vector sum.
An oscilloscope is usually used to study the instantaneous voltages on rf lines.
Since two waves of voltage are moving on the line, you need to know how to distinguish between the
two. The voltages moving toward the receiving end are called INCIDENT VOLTAGES, and the whole
waveshape is called the INCIDENT WAVE. The wave moving back to the sending end after reflection 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.
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. The reflected wave of current is shown by dashed lines 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, since there can be no current flow through an
open circuit.
Views B and C of figure 3-26 show the voltage and current distribution along a transmission line at a
point about 1/8 after a maximum voltage or current reaches the end of the line. Since 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.
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
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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
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