SWR and Reflected Power Calculator
Calculate Standing Wave Ratio (SWR) from impedance mismatch, and find the percentage of power reflected back.
Essential for antenna tuning.
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:
| SWR | Reflected Power | Effect |
|---|---|---|
| 1.0:1 | 0% | Perfect match |
| 1.5:1 | 4% | Excellent |
| 2.0:1 | 11% | Acceptable |
| 3.0:1 | 25% | Poor, investigate |
| 5.0:1 | 44% | Bad, risk of damage |
| 10.0:1 | 67% | Dangerous |
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