Everything about Vswr totally explained
In
telecommunications,
standing wave ratio (
SWR) is the
ratio of the
amplitude of a partial
standing wave at an antinode (maximum) to the amplitude at an adjacent
node (minimum), in an electrical
transmission line.
The SWR is usually defined as a
voltage ratio called the
VSWR, for
voltage standing wave ratio. For example, the VSWR value 1.2:1 denotes a maximum standing wave amplitude that's 1.2 times greater than the minimum standing wave value. It is also possible to define the SWR in terms of
current, resulting in the ISWR, which has the same numerical value. The
power standing wave ratio (PSWR) is defined as the square of the VSWR.
Relationship to the Reflection Coefficient
The voltage component of a standing wave in a uniform
transmission line consists of the forward wave (with amplitude
) superimposed on the reflected wave (with amplitude
).
Reflections occur as a result of discontinuities, such as an imperfection in an otherwise uniform transmission line, or when a transmission line is terminated with other than its
characteristic impedance. The
reflection coefficient is defined thus:
» against
x, in the case when ρ = 0.5, is shown below.
and
are the values used to calculate the SWR.
It is important to note that this graph does
not show the instantaneous voltage profile along the transmission line. It only shows the
maximum amplitude of the oscillation at each point. The instantaneous voltage is a function of both time and distance, so could only be shown fully by a three-dimensional or animated graph.
Practical implications of SWR
SWR has a number of implications that are directly applicable to radio use. There are, however, some serious misconceptions regarding the effects of SWR. The ARRL Technical Information Service has a good collection of articles which reveal many of these misconceptions.
SWR is an indicator of reflected waves bouncing back and forth within the transmission line, and as such, an increase in SWR corresponds to an increase in power in the line beyond the actual transmitted power. This increased power will increase RF losses, as increased voltage increases dielectric losses, and increased current increases resistive losses.
Matched impedances give ideal power transfer; mismatched impedances give high SWR and increased loss in the transmission line.
Reflected power in the transmission line is re-reflected at the transmitter, and is eventually radiated by the antenna (minus cable loss).
The higher voltages associated with a sufficiently high SWR could damage the transmitter. Solid state transmitters which have a lower tolerance for high voltages may automatically reduce output power to prevent damage. Transmitters using tube-type output stages may arc. The high voltages may also cause transmission line dielectric to break down and/or burn.
VSWR measurements may be taken to ensure that a waveguide is contiguous and has no leaks or sharp bends. If such bends or holes are present in the waveguide surface, they may diminish the performance of transmitter and receiver equipment strings. Arcing may occur if there's a hole, if transmitting at high power, usually 200 watts or more (Need reference for the power statement). Waveguide plumbing(External Link
) is crucial for proper waveguide performance. Reflected power may occur and damage equipment as well. Another cause of bad VSWR in a waveguide is moisture build-up, which can typically be prevented with silica gel or pressurization of the waveguide with dry gas.
A very long run of coaxial cable especially at a frequency where the cable itself is lossy can appear to a radio as a matched load. The power coming back is, in these cases, partially or almost completely lost in the cable run.Further Information
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