SWR and Reflected Power Calculator

SWR (Standing Wave Ratio) measures how well your antenna is matched to your transmission line and transmitter. A perfect match is 1:1. High SWR means power is being reflected back toward your transmitter, reducing efficiency and potentially damaging the final amplifier stage.

The Formulas:

Reflection Coefficient: Γ = (Z_L − Z_0) / (Z_L + Z_0)

SWR = (1 + |Γ|) / (1 − |Γ|)

Where:

• Z_L = load (antenna) impedance in ohms
• Z_0 = characteristic impedance of the feedline (typically 50Ω)
• |Γ| = magnitude of reflection coefficient (0 to 1)

Percent Power Loss from SWR:

Power reflected (%) = |Γ|² × 100

Power to antenna (%) = 100 − Power reflected (%)

Worked Example:

Antenna impedance: 75Ω. Feedline: 50Ω coax.

Γ = (75 − 50) / (75 + 50) = 25 / 125 = 0.2

SWR = (1 + 0.2) / (1 − 0.2) = 1.2 / 0.8 = 1.5:1

Power reflected: 0.2² × 100 = 4% Power delivered to antenna: 96%

A 1.5:1 SWR is excellent — most transceivers operate full power up to 1.5:1 or 2.0:1.

SWR Reference Table:

Practical Tips:

• Most modern HF transceivers fold back power above 3.0:1 SWR to protect the PA
• SWR changes with frequency: always measure at your operating frequency
• Coax length can affect the SWR reading at the radio but not at the antenna: use an antenna analyzer at the feedpoint
• High SWR in a long coax run causes additional heat loss in the cable itself