What is a Voltage Standing Wave Ratio (VSWR)?

What is a Voltage Standing Wave Ratio (VSWR)?

• A. A measure of the impedance mismatch between the transmission line and the antenna
• B. A measure of the transmitted power efficiency
• C. The ratio of forward to backward traveling waves in time domain
• D. The ratio of input to output impedance of a device

Answer: (A)

Voltage Standing Wave Ratio measures how well the feed line and the antenna are matched. When the antenna’s input impedance equals the line’s characteristic impedance, there are no reflections and the VSWR is 1. Any mismatch causes part of the incident wave to reflect back toward the transmitter, creating a standing wave along the line. VSWR quantifies that standing wave as the ratio of the maximum to minimum voltage along the line, which relates to the magnitude of the reflection coefficient Γ = (Zin − Z0)/(Zin + Z0) by VSWR = (1 + |Γ|)/(1 − |Γ|). A larger VSWR indicates more reflected power and poorer power transfer to the antenna. For example, with a 50-ohm line and a 100-ohm load, Γ ≈ 0.333, giving a VSWR of about 2:1, meaning roughly 11% of the power is reflected and not delivered to the antenna.

Voltage Standing Wave Ratio measures how well the feed line and the antenna are matched. When the antenna’s input impedance equals the line’s characteristic impedance, there are no reflections and the VSWR is 1. Any mismatch causes part of the incident wave to reflect back toward the transmitter, creating a standing wave along the line. VSWR quantifies that standing wave as the ratio of the maximum to minimum voltage along the line, which relates to the magnitude of the reflection coefficient Γ = (Zin − Z0)/(Zin + Z0) by VSWR = (1 + |Γ|)/(1 − |Γ|). A larger VSWR indicates more reflected power and poorer power transfer to the antenna. For example, with a 50-ohm line and a 100-ohm load, Γ ≈ 0.333, giving a VSWR of about 2:1, meaning roughly 11% of the power is reflected and not delivered to the antenna.