Understanding VSWR (Voltage Standing Wave Ratio)

This article explains what VSWR is, defines return loss, and highlights the difference between the two. VSWR stands for Voltage Standing Wave Ratio.

When a transmission line (like a cable) is connected to a load (termination) that doesn’t perfectly match its characteristic impedance, not all the power traveling down the line is absorbed by the load. Instead, some of the power is reflected back towards the source. This reflected power interacts with the forward-going (incident) signal, creating a voltage standing wave pattern along the transmission line.

The VSWR is the ratio of the maximum voltage to the minimum voltage in this standing wave pattern.

A VSWR of 1:1 means there’s no reflected power – all the power is being absorbed by the load. This is the ideal situation, but it’s rarely achieved in practice. In most real-world scenarios, a VSWR of 1.2:1 (or simply 1.2) is considered excellent.

In environments like EMC (Electromagnetic Compatibility) labs, where tests are often performed over a wide range of frequencies, a VSWR of 2.0 or higher isn’t unusual. At a VSWR of 2.0, approximately 10% of the power is reflected back to the source.

A high VSWR isn’t just about wasted power; the reflected power can also cause problems, such as overheating cables or causing amplifiers to “fold-back” (reduce their output power).

Improving VSWR

Fortunately, there are ways to improve the VSWR of a system. One method involves using impedance matching devices where impedance changes occur. Baluns, for example, are often used with antennas to convert between balanced and unbalanced signals and to match the impedance of the source to the antenna.

In EMC testing, it’s common practice to include attenuators at points where impedance mismatches are likely. For instance, some emissions standards specify using an attenuator at the connector of a biconical antenna because these antennas often have high VSWR at certain frequencies. Some conducted immunity standards suggest using a 6dB attenuator (pad) at the input of the coupling device, which is commonly 150 ohms.

While attenuators introduce power loss, they reduce VSWR by providing a better-looking termination to the signal.

Measuring and Calculating VSWR

There are various ways to measure or calculate VSWR. Historically, with open transmission lines, the voltage could be measured along the line’s length to find the maximum and minimum voltage values (which are spaced 1/4 wavelength apart). This is where the term “Voltage Standing Wave Ratio” comes from.

VSWR can be calculated using the following formula:

$VSWR = \frac{E_{max}}{E_{min}} = \frac{E_i + E_r}{E_i - E_r}$

Where:

• $E_{max}$ = Maximum measured voltage
• $E_{min}$ = Minimum measured voltage
• $E_i$ = Incident wave amplitude in volts
• $E_r$ = Reflected wave amplitude in volts

VSWR and Return Loss: The Connection

Return Loss and VSWR are directly related. Return Loss is a measure of how much power is reflected back from the load, typically expressed in decibels (dB). A higher return loss indicates a better match and less reflected power.

The following equations are used to convert between return loss and VSWR:

• $VSWR = \frac{1 + 10^{\frac{RL}{20}}}{1 - 10^{\frac{RL}{20}}}$
• $Return Loss = 20 * Log_{10}(\frac{VSWR+1}{VSWR-1})$