Standing waves.

Standing wave.
When energy is reflected back towards the source, the combination of the forward and reflected signals create a third wave. The picture on the right is a rough demonstration of the creation of a standing wave.
The forward signal (black) combines with the reflected (blue) to create the standing wave (green) which, although stood still, is continually varying in amplitude.


Standing wave ratio.
This is the ratio between the amplitude of the node and the peak amplitude of the adjacent antinode of a standing wave. A node is a point where the amplitude has least change, an antinode is a point where the amplitude sees maximum swing.
This is shown in the diagram on the right, where: As reflected signal increases, the antinode amplitude increases while the node is reduced - resulting in a higher ratio. At 100% reflection, the node is zero making the ratio equal to infinity.
Test instruments do not directly measure the standing wave, it is calculated from other measurements. Methods are shown below.

Measuring with the SWR meter.
A typical SWR meter will measure forward and reflected signals.
From these, it is possible to calculate the node and antinode values. As SWR is the ratio of these two figures, it may be calculated as shown.
The scale printed behind within the meter is designed to give you the reading after this calculation is done.

Measuring SWR with a power meter.
Another method is to use forward and reflected power figures to calculate the ratio. This one may be more useful, as you can measure forward and reflected power by reversing the power meter coax connections:
Measuring SWR from impedances.
As the standing wave ratio and impedance ratio are equal, it can also be obtained by measuring the transmitter and antenna impedance then dividing the larger figure by the smaller one:

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