Common SWR Issues
Let’s review the most common problems that cause your SWR meter to register danger on all channels: poor grounds, a short in the coaxial connectors, or an improperly installed mounting stud.
1. A large percentage of high SWR readings are caused by ground plane problems. It's a good idea to run ground straps from the body of your vehicle to the frame, doors, trunk -- everything except your dog. Running the shortest possible ground strap from the antenna to the chassis or your vehicle is generally a good solution for ground plane problems. Simply put, grounding everything that can be ground together will improve ground plane.
2. It is essential that your mount is properly grounded. Most improperly grounded mounts are connected to places on your vehicle that themselves are not thoroughly grounded. Any part of your vehicle that has a plastic or nylon bushing separating it from the chassis is probably not grounded. Also, chassis paint can often prevent a mount from being properly grounded. You can check the grounding of suspect parts with a voltage meter.
3. A short in the coaxial connectors may also be the culprit behind abnormally high SWR readings. Issues with the coaxial cables are often identifiable by eye, such as severe bends or pinches. You should know that it's essential to use 50-ohm coax for single antennas and 75-ohm for dual. When all else fails, sometimes it's necessary to replace the coax cable because there's a failure inside the line.