SWR
| SWR | |
|---|---|
| Stands for | Standing wave ratio |
| What it measures | How well the antenna's impedance matches the feed line and transmitter |
| Perfect match | 1:1 |
| Generally fine | Up to about 2:1 |
| Trouble | Above 3:1; most radios reduce power to protect themselves |
| Measured with | An SWR meter, or an antenna analyzer |
SWR, the standing wave ratio, is the number every ham checks after putting up an antenna. It describes how well the antenna is matched to the transmitter and feed line. When the match is perfect, all the power traveling up the coax is accepted by the antenna and the SWR is 1:1. When the antenna's impedance differs from the 50 ohms the system expects, some power is reflected back toward the radio, and the forward and reflected waves combine into a standing wave on the line. The worse the mismatch, the higher the ratio.
What the numbers mean
| SWR | Power reflected | Verdict |
|---|---|---|
| 1:1 | 0% | Perfect |
| 1.5:1 | 4% | Excellent |
| 2:1 | 11% | Fine for most purposes |
| 3:1 | 25% | Radio will probably reduce power; find the cause |
| 5:1 | 44% | Something is wrong |
| ∞ | 100% | Open or shorted feed line |
A reflected 11 percent sounds bad but is only half a decibel, less than anyone can hear on the other end. The real reasons to care about high SWR are that modern solid-state transmitters fold back their power to protect the output transistors, and that in coax the reflected power is partly lost as heat on its way back and forth.
Measuring it
An SWR meter sits between the radio and the antenna and reads forward and reflected power while you transmit a steady carrier at low power. Most HF radios have one built in. An antenna analyzer or a NanoVNA does the job without transmitting at all, sweeping the band and plotting SWR against frequency, which shows at a glance whether a dipole is too long or too short.
Measure at the radio end for convenience and at the antenna end for truth. Coax loss makes SWR look better at the radio than it is at the antenna.
Fixing it
- Trim or lengthen the antenna until the SWR minimum lands on the frequencies you use. A resonant antenna is the best fix.
- Check the feed line and connectors. A high SWR on every band usually means a bad connector, water in the coax or a broken element, not a tuning problem.
- Use an antenna tuner. A tuner between the radio and the feed line makes the radio see 50 ohms, so it delivers full power, but it doesn't change the SWR on the coax beyond it. For a mildly mismatched antenna on a short run of good coax, that's fine; for a badly mismatched antenna on a long run, the loss stays.
- Add a choke or balun at the feed point if common-mode current on the outside of the coax is confusing the reading.