Voltage Standing Wave Ratio (VSWR – sometimes pronounced “Viz-Wer”) or Standing Wave Ratio (SWR) is a measure of how well matched the impedance of the radio (transmitter or receiver) and antenna and transmission line carrying Radio Frequencies (RF). It is the ratio of the maximum and minimum voltage levels along the entire length of the transmission line and antenna.
Meaning of Impedance?
An impedance is a complex value (Z) that consists of many variables.
- Resistance (R) – this does not change with frequency
- Reactance (X) – this does change with frequency and consists of Inductance (L) and Capacitance (C)
It is important to remember that impedance (Z) does change with frequency.
Reflection can be quantified by means of a coefficient of Reflection (Γ), which is the function of the impedance load (ZL) and the source impedance (Z0).
Γ can then be used to calculate VSWR
A perfectly tuned antenna systems has a VSWR of 1.0, this means that no power is being reflected back along the transmission line to the radio. However, as you can see in the VSWR table, having a VSWR of just 3 means that 25% of the Tx power is being reflected back to the radio transmitter. A VSWR of 6 and over 50% is going straight back in to the radio.
The typical impedance of a transmission systems is 50Ω or 75Ω.
An impedance mismatch can result in poor performance and high reflected power back in to the radio. The transmitter might not be able to transmit at full power and could result in damage.
Factors affecting VSWR
- Antenna ground
- Nearby metal objects
- Type of antenna construction
Matching of the system impedance can be achieved by trimming (or tuning) the antenna for the correct frequency or by automatic tuning equipment.
In summary, VSWR of less than 2.0 is generally considered acceptable and little would be gained from trying to achieve the perfect 1.0!
The Wikipedia article has more in depth explanation https://en.wikipedia.org/wiki/Standing_wave_ratio