Standing-wave ratio (SWR) is a mathematical expression of the non-uniformity of an electromagnetic field (EM field) on a transmission line such as coaxial cable. Usually, SWR is defined as the ratio of the maximum radio-frequency (RF) voltage to the minimum RF voltage along the line. This is also known as the voltage standing-wave ratio (VSWR). The SWR can also be defined as the ratio of the maximum RF current to the minimum RF current on the line (current standing-wave ratio or ISWR). For most practical purposes, ISWR is the same as VSWR.

Under ideal conditions, the RF voltage on a signal transmission line is the same at all points on the line, neglecting power losses caused by electrical resistance in the line wires and imperfections in the dielectric material separating the line conductors. The ideal VSWR is therefore 1:1. (Often the SWR value is written simply in terms of the first number, or numerator, of the ratio because the second number, or denominator, is always 1.) When the VSWR is 1, the ISWR is also 1. This optimum condition can exist only when the load (such as an antenna or a wireless receiver), into which RF power is delivered, has an impedance identical to the impedance of the transmission line. This means that the load resistance must be the same as the characteristic impedance of the transmission line, and the load must contain no reactance (that is, the load must be free of inductance or capacitance). In any other situation, the voltage and current fluctuate at various points along the line, and the SWR is not 1.

When the line and load impedances are identical and the SWR is 1, all of the RF power that reaches a load from a transmission line is utilized by that load. When the load is an antenna, the utilization takes the form of EM-field radiation. If the load is a communications receiver or terminal, the signal power is converted into some other form, such as an audio-visual display. If the impedance of the load is not identical to the impedance of the transmission line, the load does not absorb all the RF power (called forward power) that reaches it. Instead, some of the RF power is sent back toward the signal source when the signal reaches the point where the line is connected to the load. This is known as reflected power or reverse power.

The presence of reflected power, along with the forward power, sets up a pattern of voltage maxima (loops) and minima (nodes) on the transmission line. The same thing happens with the distribution of current. The SWR is the ratio of the RF voltage at a loop to the RF voltage at a node, or the ratio of the RF current at a loop to the RF current at a node. In theory, there is no limit to how high this ratio can get. The worst cases (highest SWR values) occur when there is no load connected to the end of the line. This condition, known as an unterminated transmission line, is manifested when the end of the line is either short-circuited or left open. In theory, the SWR is infinite in either of these cases; in practice, it is limited by line losses, but can exceed 100. This can give rise to extreme voltages and currents at certain points on the line.

The SWR on a transmission line is mathematically related to (but not the same as) the ratio of reflected power to forward power. In general, the higher the ratio of reflected power to forward power, the greater is the SWR. The converse is also true. When the SWR on a transmission line is high, the power loss in the line is greater than the loss that occurs when the SWR is 1. This exaggerated loss, known as SWR loss, can be significant, especially when the SWR exceeds 2 and the transmission line has significant loss to begin with. For this reason, RF engineers strive to minimize the SWR on communications transmission lines. A high SWR can have other undesirable effects, too, such as transmission-line overheating or breakdown of the dielectric material separating the line conductors.

In some situations, such as those encountered at relatively low RF frequencies, low RF power levels, and short lengths of low-loss transmission line, a moderately high SWR does not produce significant SWR loss or line overheading, and can therefore be tolerated.