VSWR (Voltage Standing Wave Ratio)

Voltage Standing Wave Ratio (VSWR) quantifies how efficiently radio frequency (RF) power is transmitted from a source through a transmission line to a load, such as an antenna. It reflects the degree of impedance matching and is expressed as a ratio (e.g., 1.2:1).

A VSWR of 1:1 indicates a perfect match with no reflected power. Higher values signal greater mismatch and increased reflection.

Mathematical Calculation

VSWR is calculated from the reflection coefficient (Γ):

VSWR = (1 + |Γ|) / (1 - |Γ|)

where:

• Γ = reflection coefficient (unitless)

• |Γ| = magnitude of the reflection coefficient

The reflection coefficient itself is based on impedance mismatch:

Γ = (Z_L - Z_0) / (Z_L + Z_0)

where:

• Z_L = load impedance (Ω)

• Z_0 = characteristic impedance of the transmission line (Ω)

Alternatively, VSWR can be expressed using forward and reflected power:

VSWR = (√(P_forward / P_reflected) + 1) / (√(P_forward / P_reflected) - 1)

Significance and Practical Applications

• A VSWR of 1.5:1 in a 50-ohm system suggests a mismatch where Z_L ≈ 75 Ω or 33.3 Ω, reflecting about 4 % of the power.

• VSWR < 1.5:1 is typically acceptable for low-loss systems.

• VSWR > 2:1 can result in significant power loss, signal distortion, or transmitter stress.

VSWR is crucial in:

• Telecommunications – Ensures efficient RF power transfer

• Broadcast systems – Minimizes standing waves on antenna lines

• Radar and RF modules – Prevents hardware degradation due to reflection

Engineers use VSWR measurements to assess and optimize antenna matching, cable assemblies, and transmission line performance.