Antenna Warning Lights on CB Radio's
In our experience, the antenna warning lights on CB radios have little, if any, value at all. We have seen them light up on systems with SWR below 1.5:1 (tested with an expensive external meter) and we have seen them not light up on systems where the antenna was dead-shorted to the grounded antenna mount. Furthermore, it is all but impossible to tune an antenna system using just the antenna warning light because it is impossible to know if the system is electrically long or electrically short without knowing the exact amount of reflective power on a few fixed frequencies (channels), typically CH1, CH19 and CH40.
The only time we would put even a minute trust in the antenna warning light would be after the antenna system was tested and the antenna tuned using an external SWR meter. Then, assuming that the system was below 2.5:1 on all channels (again using data from an external SWR meter) and that the light did not come on after verifying the SWR, but later, came on during use. That could be an indication that something may have changed (corrosion, broken coax, antenna damaged from hitting something, etc.). If that was the circumstance, we wouldn't recommend transmitting with the radio until the SWR and the system was rechecked with an external SWR meter.
We do not necessarily have a lot of faith in any of the built-in radio SWR meters but they are generally much more reliable than the trouble prone antenna warning lights.
Note: Even with high SWR, it is generally okay to just listen (receive) to your CB. High SWR will limit the quality of reception but it is during transmit that damage may occur to the radio circuitry.
Updated August 7, 2014