SWR / VSWR (Standing Wave Ratio)
SWR is the measure of impedance mismatch on the line, and 1.0:1 indicates a perfect match. The proportion of power returned rises with SWR: roughly 4% at 1.5:1, 11% at 2:1, 25% at 3:1. The real risk is not this loss but the returned power thermally stressing the transmitter output stage.
On a mismatched line the forward and reflected waves superimpose to form a fixed amplitude pattern; SWR is the peak/trough ratio of that pattern.
- Viewed as loss, SWR is exaggerated: a 2:1 SWR means only roughly 0.5 dB of mismatch loss, and its effect on range is small.
- Viewed from the hardware side it is serious: the returned power is dissipated as heat in the output stage.
- Running without an antenna is the worst case; all of the power comes back. Do not key the transmitter without an antenna.
The typical causes of degraded SWR in the field are these: a broken or snapped antenna, a crushed coaxial cable, a loosened or oxidised connector, an antenna for the wrong band. When range drops, all of these can be checked by eye and by hand. On a system whose antenna and connector are sound, an SWR problem is not expected; it is more productive to look for the cause of range loss first in the layout and the noise floor.
Context in which this term is used: 433 MHz or 2.4 GHz?