___________________________________________________ Principles of Transmission Lines
3-107. In waveform (5) of figure 3-27, the incident wave has moved another
45 degrees to the right and the reflected wave 45 degrees to the left. The
resultant again is maximum negative at the end and positive maximum
1/2 wavelength from the end. The maxima are lower than those in waveform
(4). In waveform (6), the incident and reflected wave have moved another
1/8λ. The two waves again are 180 degrees out of phase, giving a resultant
wave with no amplitude. The incident and reflected waves continue moving
in opposite directions, adding to produce the resultant waveshapes shown in
waveforms (7) and (8). Notice that the maximum voltage in each resultant
wave is at the end and 1/2λ from the end.
3-108. Study each part of figure 3-27 carefully and you will get a clear
picture of how the resultant waveforms of voltage are produced. You will also
see that the resultant voltage wave on an open-ended line is always zero at
1/4λ and 3/4λ from the end of the transmission line. Because the zero and
maximum points are always in the same place, the resultant of the incident
and the reflected wave is called a standing wave of voltage.
3-109. The right-hand column in figure 3-27 shows the current waveshapes
on the open-ended line. Because the current is reflected out of phase at an
open end, the resultant waveshapes differ from those for voltage. The two
out-of-phase components always cancel at the end of the transmission line, so
the resultant is always zero at that point. If you check all the resultant
waveshapes shown in the right-hand column of figure 3-27, you will see that
a zero point always occurs at the end and at a point 1/2λ from the end.
Maximum voltages occur 1/4λ and 3/4λ from the end.
3-110. When an AC meter is used to measure the voltages and currents
along a line, the polarity is not indicated. If you plot all the current and
voltage readings along the length of the line, you will get curves like the ones
shown in figure 3-28. Notice that all are positive. These curves are the
conventional method of showing current and voltage standing waves on RF
lines.
Figure 3-28. Conventional Picture of Standing Waves
3-111. When an RF line is terminated in a short circuit, reflection is
complete, but the effect on voltage and current differs from that in an open-
ended line. Voltage is reflected in opposite phase, while current is reflected in
phase. Again refer to the series of pictures shown in figure 3-27. However,
this time the left column represents current, because it shows reflection in
phase; and the right column of pictures now represents the voltage changes
on the shorted line, because it shows reflection out of phase.
3-35
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