Excellent concept of VSWR : VSWR is a mathematical expression of the non equalization of the electromagnetic field on a wave line or RF voltage on a transmission line such as a coaxial cable or a connecting device. A standing wave pattern is generated when part of the energy of the forward traveling wave is reflected at a point where there is a constraint mismatch with the transmission path. This causes the forward and reflected wave and connects in and out with the length of the phase transmission path.
VSWR is usually defined as the ratio of the maximum RF voltage to the minimum RF voltage along the line. VSWR occurs when all devices responsible for transmitting radio signals do not match impedance wise. This includes cables, connectors and antenna. The two numbers relate to an impedance mismatch against a correct impedance match. The second number is always one. The lower the first number, the better the impedance match.
What is VSWR?
While undertaking RF design, it is often necessary to calculate VSWR or return loss, or to maintain or use RF equipment. It is sometimes useful to convert between return loss, VSWR and voltage reflection coefficient. Return loss (dB) is defined as the ratio of incoming signal to the same reflected signal as it enters a component.
Return loss (RL) can also be explained as the difference between the power of the transmit signal and the power of the signal reflections due to changes in the link and channel impedance. It means ratio of the maximum RF voltage to the minimum RF voltage along the line is called the VSWR.
VSWR (known voltage standing wave ratio), how efficiently the radio-frequency power is from a power source, through the transmission line, into the load (for one, one) through the power amplifier transmission line, A antenna).
A return loss plot indicates how well the link and the channel matches its rated impedance over a range of impedance frequencies. Higher return loss values mean a closer impedance match, which results in greater discrimination between the strengths of the transmitted and reflected signals.
Return Loss (dB) = 10 * LOG10 (Reflected Power / Incident Power)
When designing or building electronic circuits using radio frequency elements, the loss return is of considerable importance as are voltage standing wave ratios- VSWR, and reflection coefficients for the signal. This can be very important when using or designing RF equipment.
It is consider as a defeated returned (dB) and it is defined as the ratio of the incoming signal to the same reflected signal as it enters a component. Return loss (RL) can also be interpreted as the difference between the power of a transmitted signal and the power of some consideration because of changes in signal link and channel impedance.
A return loss plot indicates how well the link and the channel matches its rated impedance over a range of impedance frequencies. Higher return loss values mean a closer impedance match, which results in greater discrimination between the strengths of the transmitted and reflected signals.
Difference between VSWR and SWR
The terms VSWR and SWR are often seen in the literature about standing waves in RF systems, and ask about many differences.
SWR
SWR stands for Standing Wave Ratio. SWR explain as the voltage and current standing waves that appear on the line. This is a general description for both current and voltage waves. It is often used in conjunction with the meter used to detect the standing wave ratio. For a given mismatch both current and voltage rise and fall by the same ratio.
VSWR
The VSWR or voltage standing waveform ratio is particularly applicable to voltage standing waves that are installed on a feeder or transmission line. Since voltage steep waves are easier to detect, and in many cases the voltages are more important in the context of the breakdown of the device, the term VSWR is often used, especially within RF design areas.
Steep waves of power are also seen for some time. However this is a complete collapse because the forward and reflected power is constant (assuming no feeder losses) and the power does not rise and fall in the same way as the voltage and current standing waves which are the sum of both the forward and reflected elements.
Effects of VSWR
There are many ways in which VSWR affects the performance of a transmitter system, or any system that can use RF and matched impedance. Although the term VSWR is commonly used, both voltage and current standing waves can cause issues. Some effects are given below:
Transmitter power amplifiers may suffer: Viewed as a result of increased levels of voltage and current waves at the feeder, the output of the transmitter may damage the transistor. Semiconductor devices are very reliable if operated within their specified limits, but the voltage and current standing waves at the feeder can cause catastrophic losses if they cause it to operate outside their limits.
PA protection reduces the output power: Given the very real danger of high SWR levels, the power amplifier is harmed, with many transmitters incorporating safety circuitry that reduces the output from the transmitter as the SWR rises. This means that a poor SWR between the feeder and the antenna results in a higher SWR causing the output to decrease and hence a significant loss in transmitted power.
High voltage and current levels can harm the feeder: It is possible that a feeder may suffer damage due to the high voltage and high level current of the wave. Although in most cases the feeders will operate well within their limits and must be able to adjust the doubling of voltage and current, there are some circumstances when damage may be caused. The current maxima can cause excessive local heating that can distort or melt the plastic used, and high voltages are thought to be the cause in some circumstances.
Delays due to reflections can cause distortion: When a signal is reflected by mismatch, it is reflected back towards the source, and then may be reflected back to the antenna again. A delay equal to twice the transmission time of the signal is introduced with the feeder. This can cause inter-symbol interference if data is being transmitted, and in another instance where analog television was being broadcast, a "ghost" image was seen.
Signal reduction compared to fully matched system: Interestingly the signal level reduction due to a poor VSWR is not nearly as great as some might imagine. Any signal reflected by the load is reflected back into the transmitter and the signal may be able to be reflected back to the antenna again as matched on the transmitter, the losses that have been introduced are originally introduced by the feeder. A 30-meter length of RG213 with a loss of about 1.5 dB at 30 MHz as a guide would mean that an antenna working with VSWR would be just over 1dB at this frequency compared to a fully matched antenna will harm.
In simple word we can say that-
• A significant decrease in the amplitude of the transmitted RF signal.
• The power reflected upon burning the electronics of the reflector if they are protected from the power not being returned.
Several methods of changing or eliminating the effects of VSWR include:
• Proper use of appropriate equipment.
• Tight connections between cables and connectors.
• Use of impedance matched hardware.
• Use of high-quality equipment with calibration reports.
• keep in your mind that don't use 75 ohm cable with 50 ohm devices.
Last word
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