Standing wave ratio
SWR measures impedance matching via standing wave amplitude ratio.
Standing wave ratio (SWR) is a metric used in radio engineering and telecommunications to assess how well a load's impedance matches the characteristic impedance of a transmission line or waveguide. When there is an impedance mismatch, standing waves form along the line. SWR is calculated as the ratio of the wave's amplitude at an antinode (its maximum) to its amplitude at a node (its minimum).
Voltage standing wave ratio (VSWR), pronounced "vizwar," specifically compares the highest and lowest voltage levels on a transmission line. For instance, a VSWR of 1.2 means the peak voltage is 1.2 times the minimum voltage, assuming the line is at least half a wavelength long. SWR can also be defined using the maximum and minimum amplitudes of current, electric field, or magnetic field along the line; if transmission line losses are ignored, these ratios are all the same. Power standing wave ratio (PSWR) is the square of VSWR, but this term is deprecated because it does not directly correspond to actual transmitted power.
SWR is typically measured with a dedicated SWR meter. Because SWR depends on the load impedance relative to the line's characteristic impedance (which together determine the reflection coefficient), an SWR meter must be designed for the specific characteristic impedance of the line to interpret the impedance correctly. Most transmission lines in these applications are coaxial cables with impedances of 50 or 75 ohms, so most SWR meters are built for one of these values.
Checking SWR is standard practice in a radio station. While an impedance analyzer or bridge could provide the same information, an SWR meter is simpler and more robust. By measuring the magnitude of the impedance mismatch at the transmitter output, it reveals problems with either the antenna or the transmission line.
SWR is used to gauge impedance matching for radio frequency (RF) signals, especially in transmission lines connecting transmitters and receivers to antennas, as well as in RF cables for cable television. Impedance matching is achieved when the source impedance is the complex conjugate of the load impedance. The simplest and most loss-minimizing way to do this is for both the source and load to have zero imaginary impedance (pure resistances) equal to the line's characteristic impedance.
- Vswr example
- A VSWR of 1.2 means a peak voltage 1.2 times the minimum voltage along that line, if the line is at least one half wavelength long.
- Swr range
- SWR is always greater than or equal to unity.
- Common impedances
- Most transmission lines used in these applications are coaxial cables with an impedance of either 50 or 75 ohms.
- Matched swr
- A matched load would result in an SWR of 1:1 implying no reflected wave.
- Infinite swr
- An infinite SWR represents complete reflection by a load unable to absorb electrical power.
Lore & Background
In radio engineering and telecommunications, SWR is used as a measure of impedance matching of a load to the characteristic impedance of a transmission line carrying radio frequency signals. This especially applies to transmission lines connecting radio transmitters and receivers with their antennas, as well as similar uses of RF cables such as cable television connections to TV receivers and distribution amplifiers. Impedance matching is achieved when the source impedance is the complex conjugate of the load impedance. The easiest way of achieving this, and the way that minimizes losses along the transmission line, is for the imaginary part of the complex impedance of both the source and load to be zero, that is, pure resistances, equal to the characteristic impedance of the transmission line.
When there is a mismatch between the load impedance and the transmission line, part of the forward wave sent toward the load is reflected back along the transmission line towards the source. The source then sees a different impedance than it expects which can lead to lesser (or in some cases, more) power being supplied by it, the result being very sensitive to the electrical length of the transmission line. Such a mismatch is usually undesired and results in standing waves along the transmission line which magnifies transmission line losses. The SWR is a measure of the depth of those standing waves and is, therefore, a measure of the matching of the load to the transmission line.
Checking the SWR is a standard procedure in a radio station. SWR is usually measured using a dedicated instrument called an SWR meter. Since SWR is a measure of the load impedance relative to the characteristic impedance of the transmission line in use, a given SWR meter can interpret the impedance it sees in terms of SWR only if it has been designed for the same particular characteristic impedance as the line. Most SWR meters correspond to either 50 or 75 ohms.
Reader's Guide
SWR is significant because it provides a simple, robust measure of impedance matching in RF systems, directly indicating the presence and severity of reflected waves. A good SWR (near 1:1) implies a transmitter's output seeing the exact impedance it expects for optimum and safe operation. The SWR directly corresponds to the magnitude of the reflection coefficient, and since the magnitude of the reflection coefficient always falls in the range [0,1], the SWR is always greater than or equal to unity. Neglecting transmission line losses, the SWR measured at any point along the transmission line obtains an identical reading. By sampling the complex voltage and current at the point of insertion, an SWR meter is able to compute the effective forward and reflected voltages on the transmission line for the characteristic impedance for which it has been designed. Some SWR meters also display the forward and reflected power. The legacy of SWR is its enduring role as a standard diagnostic tool in radio stations and telecommunications, enabling operators to quickly assess antenna and transmission line health without requiring a full impedance analysis.
Did You Know?
- Voltage standing wave ratio (VSWR) is pronounced 'vizwar'.
- Power standing wave ratio (PSWR) is defined as the square of the VSWR, but this deprecated term has no direct physical relation to power actually involved in transmission.
- In the special case of a purely resistive load unequal to the characteristic impedance, the SWR is given simply by their ratio, with the ratio or its reciprocal chosen to obtain a value greater than unity.
- A quarter-wave matching section can be used to improve the match between an otherwise mismatched source and load.
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