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Troubleshoot a High SWR
If you have a high SWR on channel 1, but not on channel 40, that is easy to fix. A high SWR across all channels can been a frustrating thing to figure out. Over the years we have helped customers trouble shoot these types of problems over the phone and through email. Below is the list of steps we go through first. 99.9% of the time, a high SWR is caused by one of these:
The first thing you need to make sure of is that you are taking SWR readings correctly. We have a guide describing how to tune your CB antenna. Make sure you are not too close to buildings, trees, other vehicles, etc. when taking SWR readings. Also make sure the doors and hood are closed on the vehicle.
Insufficient Ground Plane
CB antennas need a metal surface under them to transmit correctly. You can read more on what a CB antenna ground plane is here. Make sure your antenna has as much metal under it as possible. If you are mounting on a mirror or other bracket that extends out from the vehicle try moving the mount in closer. If you just don't have much metal on the vehicle, like wiht an RV or motorcycle, you may want to go with a no-ground plane (NGP) CB antenna.
Another problem relating to the location of a CB antenna that comes is obstructions. If your antenna is mounted down low on the vehicle, like on the bumper or behind a pickup truck's cab, the signal can bounce back to the antenna, causing a high SWR. To alleviate this, keep at least the top 12 inches of the antenna above the roof line, and move the antenna higher on the vehicle if possible. We have a CB antenna installation guide that outlines choosing a location in more detail.
Poor Electrical Ground
Keep in mind that there is a difference between the ground plane and an electrical ground. Your antenna needs both. The mount your antenna is connected to needs to have a good ground the the vehcile chassis. For metal mounts (mirror mounts, side mounts, 3 way mounts, etc) this is achieved through metal to metal contact. You can test this with a light, just as you would test any vehicle ground. If you don't have a good ground, scraping paint off where the mount contacts the vehicle can help (do this at your own risk). Running a grounding strap from the mount to the chassis can also work. For magnet mounts, the ground is formed through capacitive coupling with the metal of the vehicle (through the magnet). There really is no way to improve the ground for these. Good quality magnetic mount antennas shouldn't have this problem.
There are two places a short can occur in a CB antenna system, in the coax and in the antenna stud.
A short occurs in a CB Coax when the shield is connected to the center conductor. This can happen as a result of a defect from the factory, or a break in the cable. To test for this disconnect the coax from the CB radio and CB antenna mount. Use a multimeter to check for continuity between the center conductor and the shield. There should be none. If there is continutiy, replace the cable.
A short occurs in the antenna stud when the antenna coupling nut (where the antenna screws in) makes contact with the mount. To test for this remove the CB antenna and disconnect the coax from the stud. Use a multimeter to check for continuity between the antenna coupling nut and the mount. There should be none. If there is continutiy, check the washers for the stud.
Many CB antennas consist of a copper wire wound around a fiberglass pole. If this wire is broken anywhere, you will get a high SWR. To test for this, use a multimeter to check for continutiy between the base of the antenna (the threaded part) and the very tip. If the antenna does not have a tuneable tip, you may need to use the multimeter's proble to reach the end of the wire. If there is no continuity between the base and the tip, you may have a broken antenna. Some higher power antenna have a capacitor that will cause a multimeter to read no continuity. This is not a broken antenna. Check to see if the antenna is shunt fed.
Make sure that any slack you have in the CB coax is not coiled up. Coiling the coax up will cause the cable to mimic the coil in the antenna, creating feeback to the SWR meter. Run the coax in a figure 8 instead.
Short or poor quality CB Coax
Once or twice we have seen replacing the coax fix the problem, even though the coax was not shorted. Antenna manufacturers suggest 18 feet of coax for many antennas. Sometimes a longer or higher quality CB coax will do the trick.