What is SWR?

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. Normally, SWR is described as the ratio of the maximum radio-frequency (RF) voltage to the minimum RF voltage along the line. This is also referred to as the voltage standing-wave ratio (VSWR). The SWR might also be described 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 perfect conditions, the RF voltage on a signal transmission line is the same at all points on the line, disregarding power losses caused by electrical resistance in the line wires and weak points in the dielectric material separating the line conductors. The perfect VSWR is thus 1:1. (Frequently the SWR value is written just in terms of the first number, or numerator, of the ratio since the second number, or denominator, is always 1.) When the VSWR is 1, the ISWR is also 1.

This optimal condition can exist only when the load (for instance, an antenna or a wireless receiver), into which RF power is delivered, has an impedance the same 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 have no reactance (that is, the load must be free of inductance or capacitance). In any other situation, the voltage and current fluctuate at a variety of points along the line, and the SWR is not 1.


READ MORE