What is the voltage standing wave ratio (VSWR) in RF systems?

Voltage Standing Wave Ratio (VSWR) is a fundamental parameter that measures the efficiency of radio frequency (RF) power transmission from a source through a transmission line to a load. A simple way to measure VSWR is to find the highest to lowest voltage ratio in a standing wave created on a transmission line due to an impedance mismatch. In this FAQ, we cover the basics of VSWR, including how to measure it and the effect of a high VSWR on RF systems.

VSWR occurs when an RF system’s source, transmission line, and load have impedance mismatches. In an ideal system with perfect impedance matching (typically 50 Ω or 75 Ω), VSWR equals 1, indicating 100% power transfer.

How is VSWR measured and calculated?

VSWR is always one or greater; a VSWR of 1 represents complete power transferred to the load. When a mismatch occurs, some power is reflected back towards the source, creating standing waves. The greater the mismatch, the higher the VSWR.

Figure 1 shows that when the impedance at the source and load matches, there is no reflected power, but an impedance mismatch leads to reflected power. The green waveform is the outgoing voltage from the source, the red waveform is the incoming voltage to the source, and the purple waveform is the VSWR. The highest and lowest voltage are 1.5 V and 0.5 V, respectively, which leads to a VSWR of 1.5/0.5=3.

Reflection properties can also help measure the VSWR. Figure 2 shows a circulator connected between the source (input) and load (output), which can be connected to an RF vector network analyzer to measure the reflection coefficient (Γ).

Once the reflection coefficient (Γ) value is obtained, it can be substituted in a standard formula (Figure 3) to measure VSWR. As mentioned in Figure 3, the reflection coefficient value is also commonly used to find reflected power (%), forward power (%), return loss (dB), and mismatch loss (dB).

Figure 3. Most commonly used formulas in calculating VSWR and other important related transmission line parameters. (Image: NorthEast RF)

Return loss and mismatch loss are also important parameters in the VSWR domain. Return loss is the difference between the forward power and the reflected power, while mismatch loss is the power loss caused by an impedance mismatch.

What are the effects of high VSWR?

A high VSWR damages transmitter power amplifiers due to increased voltage and current levels on the feedline. These increased voltage and current levels also significantly damage the feedline by overheating cables and transmission lines.

A high VSWR means that there are timing delays between the outgoing and incoming waves, which leads to signal distortions. As a result, the signal is infiltrated with increased noise and reduced signal clarity. Hence, distorted signals lead to decreased communication range and reliability and compromised antenna radiation efficiency.

Based on the formulas in the previous section, Figure 4 shows a table on how an increasing VSWR affects return loss, % power/voltage loss, reflection coefficient, and mismatch loss. For a VSWR of 1, it denotes infinity return loss, and the rest of the parameters are zero. This is the ideal condition that every RF engineer aims for.

However, in real-world cases, the VSWR values are usually higher, and a value of 1.5 or less is considered good, while those in the range of 1.5 and 2 are considered acceptable. A VSWR between 2 and 3 is a poor match; if the value exceeds 3, it leads to critical failure. The corresponding transmission line parameter values for these VSWR values are also shown in Figure 4.

Summary

VSWR is an important specification in products and components related to RF systems. An RF system can communicate well from the source to the load through the transmission line. The VSWR value also forms the basis for arriving at other important parameter values, such as return loss and mismatch loss. Operating an RF system with minimum power loss and improved signal quality is possible with all these parameters.