VSWR & Transmission Line Theory Tutorial Includes:
What is VSWR? Reflection Coefficient VSWR formulas & calculations How to measure VSWR How to use a VSWR meter Simple SWR bridge circuit What is return loss VSWR / Return Loss Table
Standing waves are an important issue when looking at feeders / transmission lines, and the standing wave ratio or more commonly the voltage standing wave ratio, VSWR is as a measurement of the level of standing waves on a feeder.
Standing waves represent power that is not accepted by the load and reflected back along the transmission line or feeder.
Although standing waves and VSWR are very important, often the VSWR theory and calculations can mask a view of what is actually happening. Fortunately, it is possible to gain a good view of the topic, without delving too deeply into VSWR theory.
Standing wave basics
When looking at systems that include transmission lines it is necessary to understand that sources, transmission lines / feeders and loads all have a characteristic impedance. 50Ω is a very common standard for RF applications although other impedances may occasionally be seen in some systems.
In order to obtain the maximum power transfer from the source to the transmission line, or the transmission line to the load, be it a resistor, an input to another system, or an antenna, the impedance levels must match.
In other words for a 50Ω system the source or signal generator must have a source impedance of 50Ω, the transmission line must be 50Ω and so must the load.
Issues arise when power is transferred into the transmission line or feeder and it travels towards the load. If there is a mismatch, i.e. the load impedance does not match that of the transmission line, then it is not possible for all the power to be transferred.
As power cannot disappear, the power that is not transferred into the load has to go somewhere and there it travels back along the transmission line back towards the source.
When this happens the voltages and currents of the forward and reflected waves in the feeder add or subtract at different points along the feeder according to the phases. In this way standing waves are set up.
The way in which the effect occurs can be demonstrated with a length of rope. If one end is left free and the other is moved up an down the wave motion can be seen to move down along the rope. However if one end is fixed a standing wave motion is set up, and points of minimum and maximum vibration can be seen.
When the load resistance is lower than the feeder impedance voltage and current magnitudes are set up. Here the total current at the load point is higher than that of the perfectly matched line, whereas the voltage is less.
The values of current and voltage along the feeder vary along the feeder. For small values of reflected power the waveform is almost sinusoidal, but for larger values it becomes more like a full wave rectified sine wave. This waveform consists of voltage and current from the forward power plus voltage and current from the reflected power.
At a distance a quarter of a wavelength from the load the combined voltages reach a maximum value whilst the current is at a minimum. At a distance half a wavelength from the load the voltage and current are the same as at the load.
A similar situation occurs when the load resistance is greater than the feeder impedance however this time the total voltage at the load is higher than the value of the perfectly matched line. The voltage reaches a minimum at a distance a quarter of a wavelength from the load and the current is at a maximum. However at a distance of a half wavelength from the load the voltage and current are the same as at the load.
Then when there is an open circuit placed at the end of the line, the standing wave pattern for the feeder is similar to that of the short circuit, but with the voltage and current patterns reversed.
It is worth noting that the distance between peaks on a standing wave pattern are half a wavelength apart and not a full wavelength as some may expect. This is because they are an interference pattern between the forward and reflected waveforms.
The definition of VSWR provides the basis for all the calculations and formulas.
The voltage standing wave ratio, VSWR is defined as the ratio of the maximum to minimum voltage on a loss-less line.
The resulting ratio is normally expressed as a ratio, e.g. 2:1, 5:1, etc. A perfect match is 1:1 and a complete mismatch, i.e. a short or open circuit is ∞:1.
In practice there is a loss on any feeder or transmission line. To measure the VSWR, forward and reverse power is detected at that point on the system and this is converted to a figure for VSWR. In this way, the VSWR is measured at a particular point and the voltage maxima and minima do not need to be determined along the length of the line.
VSWR vs SWR
The terms VSWR and SWR are often seen in the literature about standing waves in RF systems, and many ask about the difference.
- SWR: SWR stands for standing wave ratio. It describes the voltage and current standing waves that appear on the line. It is a generic description for both current and voltage standing waves. It is often used in association with meters used to detect the standing wave ratio. Both current and voltage rise and fall by the same proportion for a given mismatch.
- VSWR: The VSWR or voltage standing wave ratio applies specifically to the voltage standing waves that are set up on a feeder or transmission line. As it is easier to detect the voltage standing waves, and in many instances voltages are more important in terms of device breakdown, the term VSWR is often used, especially within RF design areas.
The term power standing waves is also seen some times. However this is a complete fallacy as the forward and reflected power are constant (assuming no feeder losses) and the power does not rise and fall in the same way as the voltage and current standing waveforms which are the summation of both forward and reflected elements.
How VSWR affects performance
There are several ways in which VSWR affects the performance of a transmitter system, or any system that may use RF and matched impedances.
Although the term VSWR is normally used, both the voltage and current standing waves can cause issues. Some of the affects are detailed below:
- Transmitter power amplifiers can be damaged: The increased levels of voltage and current seen on the feeder as a result of the standing waves, can damage the output transistors of the transmitter. Semiconductor devices are very reliable if operated within their specified limits, but the voltage and current standing waves on the feeder can cause catastrophic damage if they cause the devise to operate outside their limits.
- PA Protection reduces output power: In view of the very real danger of high SWR levels causing damage to the power amplifier, many transmitters incorporate protection circuitry which reduces the output from the transmitter as the SWR rises. This means that a poor match between the feeder and antenna will result in a high SWR which causes the output to be reduced and hence a significant loss in transmitted power.
- High voltage and current levels can damage feeder: It is possible that the high voltage and current levels caused by the high standing wave ratio can cause damage to a feeder. Although in most cases feeders will be operated well within their limits and the doubling of voltage and current should be able to be accommodated, there are some circumstances when damage can be caused. The current maxima can cause excessive local heating which could distort or melt the plastics used, and the high voltages have been known to cause arcing in some circumstances.
- Delays caused by reflections can cause distortion: When a signal is reflected by mismatch, it is reflected back towards the source, and can then be reflected back again towards the antenna. A delay is introduced equal to twice the transmission time of the signal along the feeder. If data is being transmitted this can cause inter-symbol interference, and in another example where analogue television was being transmitted, a “ghost” image was seen.
- Reduction in signal compared to perfectly match system: Interestingly the loss in signal level caused by a poor VSWR is not nearly as great as some may imagine. Any signal reflected by the load, is reflected back to the transmitter and as matching at the transmitter can enable the signal to be reflected back to the antenna again, the losses incurred are fundamentally those introduced by the feeder. As a guide a 30 metre length of RG213 coax with a loss of around 1.5 dB at 30 MHz will mean that an antenna operating with a VSWR will only give a loss of just over 1dB at this frequency compared to a perfectly matched antenna.
Key VSWR topics
There are several topics that form part of the basic overview of VSWR. A number of these allied topics are included below.
- Reflection coefficient: The reflection coefficient is a parameter that indicates the portion of the signal that is reflected at the junction of the two elements of the matched system.
Read more about . . . . Reflection coefficient.
- How to measure VSWR: One of the key concepts for any parameter is its measurement. There are several ways of measuring VSWR, one of the most popular being to use a simple SWR meter.
Read more about . . . . how to measure SWR.
- Return Loss: Return loss as a parameter is closely allied to VSWR. It is actually a measure of the signal that is reflected when compared to the incident signal - it is the loss of the reflected signal when compared to the incident, typically expressed in decibels.
Read more about . . . . Return Loss.
Standing wave ratio is an important parameter for any feeder system. Although both current and voltage standing waves are set up, it is often the voltage standing wave ratio that is more widely discussed, as a result of the fact it is easier to detect and measure.
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