Basics of Voltage Standing Wave Ratio

• Voltage standing wave ratio (VSWR) is defined as the ratio between transmitted and reflected voltage standing waves in a radio frequency (RF) electrical transmission system.
• It is a measure of how efficiently RF power is transmitted from the power source, through a transmission line, and into the load.

• A common example is a power amplifier connected to an antenna through a transmission line.
• SWR is the ratio between transmitted and reflected waves.
• A high SWR indicates poor transmission-line efficiency and reflected energy, which can damage the transmitter and decrease transmitter efficiency.
• Since SWR commonly refers to the voltage ratio, it is usually known as the voltage standing wave ratio (VSWR).

The Standing Wave

• A standing wave, also called a stationary wave, combination of two waves moving in opposite directions, each having the same amplitude and frequency.
• The phenomenon is the result of interference; that is, when waves are superimposed, their energies are either added together or canceled out.

Mathematically

• VSWR is the voltage ratio of the signal on the transmission line:
  VSWR = |V(max)| / |V(min)|
• where V(max) is the maximum voltage of the signal along the line, and V(min) is the minimum voltage along the line.

It can also be derived from the impedances:

• VSWR = (1+gamma)/(1-gamma)
  where gamma (gamma) is the voltage reflection coefficient near the load, derived from the load impedance (ZL) and the source impedance (Zo):
  gamma = (ZL-Zo)/(ZL+Zo)
  If the load and transmission line is matched, gamma = 0, and VSWR = 1.0