Return Loss is a measure of the amount of energy that is reflected back at a discontinuity in a Radio Frequency Circuit [1]. It is specified in dB.
This calculator converts Return loss (RL) to Voltage Standing Wave Ratio (VSWR).
Formula
VSWR = (1 + 10-RL/20)/(1 – 10-RL/20)
Example Calculation
A Return Loss of 10 dB is equivalent to a VSWR of 1.93.
Note that like Return Loss, VSWR has no units. It is often written as a ratio relative to 1. For example, VSWR = 1.93:1
What is Return Loss?
Return loss is the ratio between the incident and reflected energy at the input of an RF system. Mathematically it is represented as
Return Loss = Incident Power / Reflected Power
In a good design, no power is reflected back. However in actual practice, there’s always some power that is returned back to the source on account of impedance mismatch.
A good design goal is to aim for a return loss of at least 14 dB across the specified frequency range. In general return loss should be as high as possible.
Effect of Return Loss on RF Transmitter Range
Practically, if the power at the output connector of a transmitter is +30 dBm and the return loss is 3 dB, then only +27 dBm is transmitted. This lowers the transmission range significantly. For example, in the case of an FM broadcast in a rural setting, the range is lowered from 11 km (ideal) to 7.8 km (3 dB return loss). This represents nearly 30% loss in range due to mismatch at the antenna.
The goal of an antenna designer is to minimize the amount of reflected power for a given level of incident power. In other words to maximize the value of Return Loss.
The value of return loss for an antenna will vary with frequency. An antenna designed for the FM band should have excellent (high) return loss in the range of frequencies 88 MHz to108 MHz. It may have poor values of return loss outside this band.
Can Return Loss be Negative?
No, it cannot. Return loss is always a positive number.
What is VSWR?
Voltage Standing Wave Ratio (VSWR) is another measure of the efficiency of power transmission from a source to the load.
The minimum value of VSWR is 1. In this case all the power from a source is absorbed by the load. A VSWR of 10 is equivalent to a return loss of 1.75 dB.
A good value of VSWR is less than 1.5. This is equivalent a Return Loss of 14 dB or higher.
References
[1] Return Loss on Wikipedia