Voltage standing wave ratio (VSWR) and return loss

RF power transfer and impedance matching

For maximum RF power transfer, the impedance of the source and load must be matched. In most RF systems, this standard impedance is 50 ohms, though 75 ohms is also common in applications like cable television. When impedances are matched, all RF power is transferred from the source to the load with minimal reflection.

However, real-world devices often introduce complex impedances, comprising resistive (real) and reactive (imaginary) components. These components can vary with frequency, especially in devices like antennas, where impedance changes significantly across their operating range.

Impedance mismatches result in reflected power traveling back toward the source. This reduces system efficiency and potentially damages components like amplifiers. Reflections can also degrade signal quality, create standing waves and generate heat, leading to performance issues and shorter component lifespans. In high-frequency systems, mismatches can significantly alter circuit behavior and complicate measurements.

Return loss vs. VSWR

Reflected power is quantified relative to forward power using two primary metrics: return loss and VSWR.

• Return Loss: Return loss quantifies how much power is reflected back toward the source due to impedance mismatches in the transmission line. It measures the ratio of the incident power to the reflected power, expressed in decibels (dB). If the incident power and the reflected power are expressed in 'absolute' decibel units, (e.g., dBm), then the return loss in dB can be calculated as the difference between the incident power (in absolute dBm units) and the reflected power (also in absolute dBm units).
  Higher return loss values indicate lower levels of reflection, which is desirable for efficient power transfer.Larger values of return loss indicate lower levels of reflection, which is desirable for efficient power transfer. For instance, a forward power of 50 dBm and reflected power of 10 dBm result in a return loss of 40 dB.

• VSWR: VSWR represents the ratio of the maximum to minimum voltage in the standing wave formed by the interference of forward and reflected waves. A perfect match yields a VSWR of 1, indicating no reflected power. Higher VSWR values correspond to greater reflections. For example, a VSWR of 1.5 reflects only 4% of the forward power, whereas a VSWR of 6 reflects 50%.

Solutions for high VSWR and low return loss

High VSWR can lead to significant inefficiencies and risks:

• Energy loss: Increased reflections reduce the power delivered to the load.
• Equipment damage: Excessive reflected power can harm sensitive components in the RF source.

To mitigate these issues, two common approaches are employed:

Matching networks: These networks adjust the load impedance using capacitive and inductive elements, transforming it to match the source impedance. This technique is particularly effective in applications with frequency-dependent impedances, like antennas.

Foldback protection: By reducing forward power during high-reflection conditions, foldback mechanisms protect the RF source from damage. This is often implemented in high-power systems, such as broadband amplifiers.