What is SWR (VSWR), and what does high SWR lead to?
SWR is the measure of impedance mismatch between the antenna and the transmitter. 1.0:1 is a perfect match; at 1.5:1 roughly 4 percent of the power returns, at 2.0:1 11 percent, at 3.0:1 25 percent. A 2:1 SWR means only roughly 0.5 dB of loss; the real risk is not the loss but the returned power stressing the transmitter's output stage.
Taking SWR to be the main cause of a drop in range is a common mistake. Look at the numbers: a 2:1 mismatch returns only 11 percent of the power, which is about half a dB of loss and makes no noticeable difference to range. In the same system, wrong polarisation can cost 20 dB and an antenna inside the enclosure 30 dB. Look at those first.
The real importance of SWR is on the hardware side:
- The returned power produces heat and voltage stress in the output transistor; in continuous operation it shortens the life.
- A poor match causes the feed line itself to radiate; the radiation pattern becomes unpredictable.
- A mismatch is most of the time the symptom of another fault: a broken antenna, a loose connector, a water-logged cable, an antenna of the wrong length.
The typical causes of raised SWR are an antenna that has been cut or chosen for the wrong frequency, a missing ground plane, the antenna being mounted too close to metal, and a damaged coaxial cable. If you cannot measure SWR in the field, checking the antenna and connector by eye and then swapping in a known good antenna and comparing the range is the fastest method.
Full topic: Wireless remote control installation: pairing, relay, safety