Troubleshoot a High SWR

When you are experiencing a high SWR reading on one end of the frequency range, it is easy to adjust your antenna up or down to correct the problem. A high SWR across all channels can be a little more frustrating to figure out. Over the years we have helped customers trouble shoot these types of problems over the phone and through email. We have found that when you are trouble shooting a high SWR the list below will aid you in fixing your problem 99.9% of the time.

Troubleshoot High SWR

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Location

And by this, we mean where your vehicle is when you are taking readings. The first thing you need to make sure of is that you are taking SWR readings correctly. If you are not sure, check our 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.

Short

The most common cause of a high SWR is an antenna shorted to ground. There are two places a short can occur in a CB antenna system, in the coax and in the antenna stud.

Coax

A short occurs in a CB Coax when the shield is making contact with 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 continuity, replace the cable.  Click here for information on testing CB Coax

Stud

A short occurs in the antenna stud when the antenna coupling nut (where the antenna screws in), or the bolt, makes contact with the mount. There are nylon washers that prevent this, but they are sometimes left out or installed improperly.  The whole purpose of an antenna stud is to isolate the antenna from ground. When the nut or bolt contact the mount, this fails. To test for an incorrectly installed stud 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 continuity, check the nylon washer placement on the stud.

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 to the vehicle 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 or multimeter just as you would test any vehicle ground. See our guide for testing a CB antenna ground for more information. 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 on magnet mounts. Good quality magnetic mount antennas shouldn't have a grounding problem.

Coiled Coax

Often times, especially when you are using the factory recommended 18 feet of coax, you will have extra coax between the antenna and CB radio. 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 signal feedback. Run the coax in a large figure 8 or purposely run the coax in a longer path to the radio.

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 with an RV or motorcycle, you may want to go with a no-ground plane (NGP) CB antenna.

Obstructions

Another problem occurs when obstructions prevent your antenna from radiating well. 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 position the antenna as high as possible on the vehicle. We have a CB antenna installation guide that outlines choosing a location in more detail.

Broken antenna

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 continuity between the base of the antenna (the threaded part) and the very tip. If the antenna does not have a tunable tip, you may need to use the multimeter probe 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.

Short or poor quality CB Coax

Once or twice we have seen replacing the coax correct the problem, even though the coax was not shorted. Antenna manufacturers recommend 18 feet of coax. Sometimes replacing your CB coax with a longer or higher quality cable will do the trick.

Antenna Mounting Location

The location you mount your antenna can cause a high SWR as well. If you mount the antenna low on the vehicle and close to the body, the signal will bounce back into the antenna rather than getting out. The only option in this case would be to raise the antenna up, or reposition the antenna. The best place to mount a CB antenna for maximum performance is where there is a flat radiating surface.  The center of the vehicle's roof is the best location.  There the antenna has 360 degree ground plane.  

CB Antenna Length

CB Antenna manufacturers make every effort to "pre-tune" antennas. It is impossible to have an antenna tuned for every scenario, though, and sometimes they are way off. If all else fails it is possible your antenna is just too long. In some instances we have cut up to 2" off of the whip portion of a center loaded coil antenna before the SWR dropped to acceptable levels. This is a last-resort.  Do it at your own risk. It is very easy to cut too much off and ruin the antenna. If you are going to try it, we recommend cutting off a small amount each time and take a new reading in between to gauge your progress.