26. How can we measure standing waves at UHF frequencies?
a. One way that we can detect standing waves is by connecting a neon
bulb across the line and sliding it along the line as in Part A of Figure
14. The neon bulb glows very brightly at the maximum voltage points. As
you move the bulb along to the minimum voltage points, it becomes dimmer or
completely dark.
Figure 14. Measuring Standing Waves of Voltage.
b. When it is necessary to measure actual voltage values, you will use
indicator. This method is shown in Part B of Figure 14. When measuring the
voltage at various places along the line, you will find that the readings
vary from some maximum value to some minimum value.
The ratio of the
maximum voltage to the minimum voltage reading is called the standing wave
ratio or SWR.
c. For example, Figure 15 shows a transmission line with standing waves
of current and voltage. The same effect occurs on both sides of the line.
Standing waves are on the line because the load is not equal to the Zo of
the line. Some of the RF energy is absorbed by the load, but most of it has
been reflected. The voltage waveforms are plotted on a scale from 0 to 3
volts.
The minimum voltage is 1 volt, and the maximum is 3 volts.
The
standing wave ratio is as follows:
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