d. An SWR of 3 indicates that the load impedance is either three times
greater than the Zo of the line or one-third the Zo of the line. In this
case, the load impedance is one-third the Zo of the line because the voltage
at the load is a minimum.
Figure 15. Measuring the Standing-Wave Ratio.
27. The SWR indicates an impedance mismatch.
a. The standing wave ratio is actually an indication of the degree of
transmission line is terminated in a resistive load equal to the Zo of the
line, the maximum and minimum values of voltage are the same.
matched condition, the SWR is 1 and the line is called a matched or flat
line. There are no reflections; all the RF energy is absorbed by the load.
A matched line is also called a non-resonant line.
b. When the load impedance is not equal to the Zo of the line, you will
get an SWR greater than 1.
The line is called a mismatched or resonant
length of line used. The SWR is highest when the line termination is either
c. The standing-wave ratio may also be defined as the ratio between the
maximum current and the minimum current.
To measure the standing wave of
current, an RF ammeter or a DC ammeter with a rectifier is used. The meter
is connected to a loop of wire which is then placed close to the lines to
Part A of Figure 16 shows how this is
done; Part B of Figure 16 shows the circuit for the meter.