What SWR measures
When a transmission line feeds a load whose impedance differs from the line’s, part of the power reflects back toward the transmitter. The forward and reflected waves add up into a standing wave along the cable. The standing wave ratio (SWR, or VSWR) compares its maximum and minimum voltage. A perfect match gives 1:1.
The formulas
The reflection coefficient Γ links everything:
- From power meters: |Γ| = √(reflected power ÷ forward power)
- From impedances: Γ = (Z_L − Z0) ÷ (Z_L + Z0), with Z_L = R + jX
- SWR = (1 + |Γ|) ÷ (1 − |Γ|)
- Return loss = −20 log10 |Γ| (dB, higher is better)
- Mismatch loss = −10 log10 (1 − |Γ|²) (dB of power not delivered)
Reference values
| SWR | Γ | Return loss | Power reflected | Mismatch loss |
|---|---|---|---|---|
| 1.0:1 | 0 | ∞ | 0% | 0 dB |
| 1.5:1 | 0.20 | 14.0 dB | 4.0% | 0.18 dB |
| 2.0:1 | 0.33 | 9.5 dB | 11.1% | 0.51 dB |
| 3.0:1 | 0.50 | 6.0 dB | 25.0% | 1.25 dB |
| 5.0:1 | 0.67 | 3.5 dB | 44.4% | 2.55 dB |
Examples
- A wattmeter shows 100 W forward and 4 W reflected: Γ = 0.2, SWR 1.5:1, return loss 14 dB.
- A 75 Ω antenna on 50 Ω coax with no reactance gives the same 1.5:1, since for purely resistive loads SWR = R ÷ Z0 or Z0 ÷ R.
What a high SWR really costs
A 2:1 SWR loses only about half a decibel at the antenna, which is barely noticeable on the air. The bigger problems are:
- Many solid-state transmitters fold back power above about 2:1 to protect their output stage.
- Reflected power raises voltage and current peaks along the line, adding cable loss and stress on connectors.
- A sudden change in SWR often points to a fault: water in a connector, a damaged cable or a broken element.
Tips
- Measure at the transmitter end, but remember that cable loss makes the SWR look better than it is at the antenna.
- An antenna tuner matches the transmitter to the line; it does not fix the mismatch at the antenna itself.
- Adjust a dipole’s length to move its lowest SWR to the frequency you use most.