Choosing a suitable RF coaxial cable requires understanding its application. Below, we will explore the consistency between standing wave ratio and impedance.

Signal reflection: VSWR and impedance consistency
 

When RF signals are input into coaxial cable assemblies, three situations occur:

(1) The signal is transmitted to the other end of the cable, as expected;
(2) When a signal propagates along a cable, it is converted into heat or leaked out of the cable, resulting in loss in both forms;
(3) Reflect back towards one end of the cable input.
The signal is reflected back along the direction of the cable input, which is caused by changes in impedance along the cable length direction. These changes include differences in impedance between cables and the devices they are connected to. Usually, connectors and the interface between connectors and cables are the main factors affecting reflection. In addition, the cable itself can also cause reflection, and one source of cable reflection is from periodic changes in impedance. And this change is generated when it is added to a specific frequency during the production process. When scanning and observing with a set of frequencies, a peak will appear.

The size of reflections can be expressed in several ways. Perhaps the familiar one is VSWR. Reflection can also be expressed as return loss, which is the ratio of reflected power to incident power. Usually expressed in dB.

Low reflected power or low voltage standing wave ratio is usually used as an indicator for coaxial components, including cables, connectors, and cable components. It indicates how well the consistency of the cable is maintained along its length, and whether the connectors are properly designed and installed; It also reflects how well the size of the connector matches the transition section of the cable diameter. It is usually a function of frequency, and as frequency increases, reflection also increases.

In many applications, low reflection power is a key factor in achieving good system performance. In this situation, selecting cables and connectors is crucial. Additionally, in order to achieve good results, it is necessary to carefully connect the connector to the cable. When purchasing cable components, VSWR's critical applications should be considered.

Note that at a specific frequency, the actual input impedance may be very different from the characteristic impedance of the cable, which is caused by reflection in the line. The VSWR of a specific length of cable is a pointer to the difference between the actual input impedance of the cable and its average characteristic impedance.