How to use the vswr, return loss and mismatch loss calculator
- Choose what you know: a VSWR reading, a return loss in decibels, a reflection coefficient, the forward and reflected power, or the load impedance as resistance and reactance.
- Enter the value or values. For a load, also enter the impedance of the line, such as 50 ohms.
- Read the VSWR, reflection coefficient, return loss, mismatch loss and the share of power that is reflected and delivered.
- Use the steps to see how each number follows from the reflection coefficient.
Formula
Reflection coefficient G = (VSWR - 1) / (VSWR + 1)
G = square root of (reflected power / forward power)
G = |(ZL - Z0) / (ZL + Z0)| for a complex load ZL = R + jX on a line Z0
Return loss = -20 log10(G); mismatch loss = -10 log10(1 - G squared)
All of these describe the same thing: how much of a wave is sent back from a load that does not match the line. The reflected share of the power is G squared.
A 2:1 mismatch and a 75 ohm load
A VSWR of 2:1 gives G = 1/3. The return loss is 9.5424 dB, the mismatch loss is 0.51153 dB, and 11.111 percent of the forward power comes back while 88.889 percent reaches the load.
A 75 ohm resistor on a 50 ohm line has G = 25/125 = 0.2 and a VSWR of 1.5:1, with a return loss of 13.979 dB. A load of 50 + j50 ohms on a 50 ohm line has G = 0.44721, a VSWR of 2.618:1 and a reflection angle of 63.435 degrees. A forward power of 100 W with 4 W reflected is also 1.5:1, with 96 W delivered.
Reading the numbers
A VSWR of 1:1 means a perfect match and no reflection. As the number grows, more power is sent back toward the source. Return loss expresses the same thing in decibels, where a larger number is better: 20 dB means only 1 percent of the power returns. Mismatch loss is the power that never reaches the load, as a loss in dB.
To turn a power ratio into decibels use the decibel calculator. For the length of a simple antenna see the dipole antenna length calculator, and for how much signal remains over a distance see the free space path loss calculator.
What the numbers at the transmitter show
A meter at the transmitter reads the mismatch after the cable has attenuated the wave twice, once going out and once coming back. A lossy cable can therefore make a poor antenna look better than it is. These values are the ideal ones at the load end, so use them with the cable loss in mind.
Assumptions and limits
- The line is lossless, so the reflection is the same at every point along it.
- A load impedance is entered as a series resistance and reactance at a single frequency.
- A short circuit or a pure reactance reflects all of the power, and VSWR is not defined for it, so the page refuses it.
- Power ratings, standing wave patterns along the line and tuner losses are not modeled.
Frequently asked questions
How do I convert VSWR to return loss?
Find G = (VSWR - 1) / (VSWR + 1) and then return loss = -20 log10(G). A VSWR of 2 gives G = 0.3333 and a return loss of 9.54 dB.
What is a good return loss?
Larger is better, because it means less reflection. A return loss of 20 dB means 1 percent of the power returns, which is a VSWR of about 1.22. The level that is good enough depends on your equipment and goals.
What is mismatch loss?
It is the part of the forward power that does not reach the load because of reflection, written as a loss in decibels: -10 log10(1 - G squared). At 2:1 it is about 0.51 dB.
Why can a reactive load give a high VSWR?
Reactance adds to the difference between the load and the line. A load of 50 + j50 ohms on a 50 ohm line has a VSWR of 2.62 even though its resistance matches, because the reactive part reflects part of the wave.