VSWR Explained

 Download this article of VSWR Explained
Here is a useful "cheat sheet" converting VSWR to the percentage of reflected power.
This online VSWR Calculator is also very useful

A Brief History of VSWR

When electric telegraphy was the dominant means of wired communication, the lines used were comprised of barecopper wires suspended on telegraph poles. For insulation the line relied on the large spacing between the wires as wellas the wires being mounted on individual glass or ceramic stand-offs as shown in Figure 1(a). These lines ran for milesand miles and were prone to damage caused by storms, fallen trees, and partial shorts due to the top of the polesmaking good nesting sites for large birds.

The lineman sent to locate and correct the fault somewhere along the miles of suspended wirescould at least identify the type of fault by examining the standing wave on the line created by thefault.

The lineman would gauge how brightly a light bulb connected across the line would glow as the connection was movedalong the line (Figure 1(b)). If the bulb lit brightly at one place on the line and would not light at all further along the line,then he knew to look along the line for an open or short circuit. If the bulb was fairly bright at one place and somewhatdimmer in another, he knew to look for a partial short across the wires.

This all seems rather primitive, but we should bear in mind that voltage measuring instruments of the day were a delicatemechanism housed in hand-made wooden cases. This made them expensive and fragile, whereas the light bulb was comparatively cheap and robust. The method was cleverer than youmight at first suppose, since it was a form of bolometer able to indicate the RMS value of the two voltage extremes on the line.

Commonplace but pretty much gone in the RF industry by the 1990s, a bolometer fed with RFwould be warmed by the RF resulting in a change in resistance in one arm of a bridge. Theoutput of the bridge gave the RMS value of the RF waveform, no matter how complex thewaveform.

At microwave frequencies slotted lines became a way of accurately determining the ratio of themaximum voltage to the minimum voltage (the VSWR, symbol 's'), and because of the simplicityof measurement and the easy math associated with it, VSWR became an everyday parameter.As the name suggests, a slotted line is a length of waveguide with a slot along the top. A probeis moved along the slot and a detector gives the voltage at any point on the line. Once you haveobtained the two extremes of voltage you can determine the ratio of the two. Once you have thisratio it is easy to calculate the reflected power coefficient, symbol rho. The reflected powercoefficient is the amount of power reflected back compared to the incident power. The AHwebsite has a calculator allowing conversion between s and rho.

Note, many textbooks portray a standing wave as shown in Figure 2.

Figure 2 is in fact a plot of the detected voltage as the probeis moved along the line This can be misleading since thestanding wave actually goes positive and negative.

What is VSWR

We continue the explanation by ignoring the Voltage and Ratio parts for now and examine how a Standing Wave is created.

The Standing Wave

Unless a test signal on a transmission line (e.g. 50 ohm coaxial cable) is terminated in 50 ohms,some of the signal will be reflected back along the line. This can best be understood by lookingat the extreme mismatch termination values, that is a short circuit (zero ohms) and an opencircuit (infinite ohms).

Short-Circuit Termination
A voltage cannot exist across a perfect short-circuit, that is the voltage can only have the value0 volts at the short-circuit. A basic law of physics is the conservation of energy. Energy cannotjust disappear, it has to accounted for somehow. Mother Nature gets around the zero voltsrequirement by creating an equal and opposite signal that travels back down the line. At theshort-circuit the +E and -E cancel each other to give the required zero volts.

Open-Circuit Termination
This is the 'dual' of the short-circuit situation. A current cannot flow in a perfect open-circuit, thatis the current can only be zero amps at the open-circuit. Again, Mother Nature gets around thezero amps requirement by creating an equal and opposite signal that travels back down the line.At the open-circuit the +I and -I cancel to give the required zero amps. Technically this shouldbe +H field and -H field, but for our purposes we will stick with +I and -I.

Creation of the Standing Wave
Figures 3(a) and 3(b) show a forward wave and a reflected wave about to 'meet' and interacton a transmission line. The box is our viewing window of the interaction as it occurs. The box isa half-wavelength wide.

Figures 4(a) to 4(m) show the standing wave as the forward and reflected waveforms overlapand add algebraically. The addition is the green trace. Close observation of the green trace inthe observation window shows it stands still, that is it pulses positive and negative, but stays inthe same place along the line. Hence the name Standing Wave

At last the acronym Voltage Standing Wave Ratio makes sense.

NOTES

Download this article of VSWR Explained
Here is a useful "cheat sheet" converting VSWR to the percentage of reflected power.
This online VSWR Calculator is also very useful