When an RF cable is mismatched, i.e. connected to a load of a different impedance to that of the cable, not all the power supplied to the cable is absorbed by the load. That which does not enter the load is reflected back down the cable.

This reflected power adds to the incident voltage when they are in phase with each other and subtracts from the incident voltage when the two are out of phase. The result is a series of voltage – and current – maxima and minima at halfwavelength intervals along the length of the line. The maxima are referred to as antinodes and the minima as nodes.

The voltage standing wave ratio is the numerical ratio of the maximum voltage on the line to the minimum voltage: VSWR = Vmax/Vmin. It is also given by: VSWR = RL/Z0 or Z0/RL (depending on which is the larger so that the ratio is always greater than unity) where RL = the load resistance.

The return loss is the power ratio, in dB, between the incident (forward) power and the reflected (reverse) power. The reflection coefficient is the numerical ratio of the reflected voltage to the incident voltage.

The VSWR is 1, and there is no reflected power, whenever the load is purely resistive and its value equals the characteristic impedance of the line. When the load resistance does not equal the line impedance, or the load is reactive, the VSWR rises above unity.

A low VSWR is vital to avoid loss of radiated power, heating of the line due to high power loss, breakdown of the line caused by high voltage stress, and excessive radiation from the line. In practice, a VSWR of 1.5:1 is considered acceptable for an antenna system, higher ratios indicating a possible defect.

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