WHAT IS SWR?
SWR is short for Standing Wave Ratio. It is a measure of how well the antenna is performing. Most problems with the antenna or antenna cable will show up during an SWR check. Please note that the SWR is a measure of antenna performance. A poor SWR is caused by the antenna, the antenna cable, or a bad antenna installation. Poor SWR is not caused by the radio. Since the SWR is a function of the antenna system, changing radios will not affect the SWR.
WHY IS SWR IMPORTANT?
SWR readings typically have a range of about 1 to 5. The lower the number, the better the antenna is working. As the SWR climbs, both transmit and receive range will drop. The SWR reading must never exceed 3, otherwise the transmitter in the CB radio can overheat and eventually fail. Excessively high SWR can also cause other problems such as a squealing noise, or can cause interference with other electronic devices in the vehicle. An SWR below 2 is very good, and anything below 1.5 is excellent.
HOW CAN I MEASURE MY SWR?
SWR is measured using an SWR meter. Some higher-end CB radios have a built-in SWR meter, which is very convenient. Other models must use an external SWR meter and a short “patch cable”. The SWR meter will have 2 connectors on the back. The antenna cable will screw onto the connector marked ANT. The patch cable will screw onto the other connector. The other end of the patch cable screws onto the CB. The patch cable needs a PL-259 connector on each end. The patch cable should be kept as short as possible, no longer than 3 feet in length.
WHERE CAN I GET AN SWR METER?
SWR meters can be found at places that sell CBs and CB accessories. This includes CB shops, truck stops, and Radio Shack. An SWR meter and patch cord from Radio Shack costs about $35 total. It doesn’t cost much for the equipment and SWR is important -- it makes sense to invest in a meter.
WHAT IS THE PROCEDURE TO CHECK SWR?
The procedure for checking SWR is basically the same whether using a built-in meter or an external meter. Follow the instructions that come with the meter. Here is the basic procedure:
- SWR must be checked in an open area. Park the vehicle away from buildings and trees, which can affect your readings. Do NOT check SWR inside a garage!
- Close the doors, hood, and trunk lid.
- Select the channel to check. It is recommended to check channels 1, 19, and 40.
- Set the switch to the CAL or FORWARD position (depending on the meter).
- Push the transmit button on the microphone, then adjust the calibrate knob so that the needle goes to the cal marker on the display. The cal marker is always located on the far right-hand side of the display (maximum deflection).
- While still holding the transmit button, set the switch to the SWR or REFLECTED position.
- Read the SWR on the meter.
NOTE: When you switch to a different channel, you should always re-check the CAL setting (repeat steps 4 through 7).
HOW IS THE SWR ADJUSTED?
The object is to get the SWR the same on channels 1 and 40, and below 3. Whatever you do, make sure the SWR is below 3. Once the SWR is about the same on channels 1 and 40 and is also less than 3, the antenna is tuned for optimum performance.
Begin by checking the SWR on channels 1 and 40, then refer to the following:
* IF THE SWR IS HIGHER ON CHANNEL 1, THE ANTENNA IS TOO SHORT.
* IF THE SWR IS HIGHER ON CHANNEL 40, THE ANTENNA IS TOO LONG.
An adjustment is made to the antenna (refer to the antenna instructions), then the SWR on channels 1 and 40 is re-checked. This process is repeated until the readings are about the same. Remember that the readings must also be below 3. When you are done, check the SWR on channel 19. It should be even less than what you got on channels 1 and 40.
Most antennas are adjustable. Refer to the instructions that came with your antenna or call the manufacturer. Here are a few methods, which vary with manufacturer:
- Antennas with a steel whip are usually adjusted by loosening a set screw and raising or lowering the whip. If you have a center-loaded antenna (fat section in the middle of the whip), adjust the top whip up and down. Even if the instructions do not refer to raising and lowering the whip, virtually all whip antennas can be adjusted in this manner.
- Some models have a metal collar on the outside of the antenna load (at the base). The collar is turned to adjust the antenna.
- Some antennas have a sleeve that slides up and down for tuning.
- Most fiberglass antennas can be adjusted as follows:
- Remove the plastic cap from the tip of the antenna.
- Slit the plastic cover to expose the top coils of wire.
- To shorten the antenna, cut off the top coil.
- To lengthen the antenna, pull up the top coil.
- Replace the plastic cap.
It is best if you refer to the specific instructions for your antenna or call the antenna manufacturer.
I CAN’T GET MY SWR DOWN
If you can’t get your SWR below 3:
A. There may be a problem with the antenna installation. Refer to the installation instructions that came with the antenna, and check your installation. Mirror-mount and trunk-lip-mount antennas typically must have good electrical contact with the frame of the vehicle. If a mirror bracket is mounted to a fiberglass door, try running a #12 ground wire from the antenna bracket to the door hinge. On a trunk-lip-mount, you may need to scrape the paint away where the set screws contact the trunk lid. You may also need to run a #12 ground wire from the trunk lid to the frame inside the trunk.
Mounting the antenna to an aluminum surface (tool box, etc.) may not provide a good ground, which can cause high SWR. This is because aluminum is not a good conductor. A grounding problem can also occur if mounting to a luggage rack. You can try running a #12 ground wire to the antenna mount, but you may need to relocate the antenna.
It is also possible that you are not using the right kind of antenna. Most CB antennas must be mounted to a metal surface (“ground plane”) in order to operate properly. The metal body of the vehicle serves this purpose. On fiberglass or plastic body vehicles, a “no-ground-plane” antenna is needed. This type of antenna is specially designed to operate without a metal surface.
B. The antenna cable has been shortened or lengthened. Changing the length of the antenna cable can affect the SWR. If your antenna came with cable, DO NOT shorten the cable. It is permissible to lengthen the cable in 3-foot increments, but DO NOT splice the cable. Use an in-line connector to connect an additional cable length.
If your antenna did not come with cable, try using an 18-foot cable. If you have extra cable, do not tightly coil the extra length. You can run it back-and-forth inside the vehicle, or coil it in at least a 12 inch diameter loop and then squeeze it together in the middle into a bow shape and tape it. You can also contact the antenna manufacturer for recommended lengths.
C. The antenna cable is damaged. This usually results in an extremely high SWR. If the needle is going all the way to the cal marker (or beyond) when you try to read the SWR, then either the cable is bad, the mount is installed incorrectly, or the antenna itself is bad. Disconnect the antenna cable from the radio and use an ohm meter to measure between the center conductor and the shell. There should be NO continuity. If you get a reading, then disconnect the cable from the antenna and repeat the test. If you still get a reading, then the cable is bad. If you no longer get a reading, then the mount is installed incorrectly.
D. The whip may be too long (steel whip antennas). If you have pushed the steel whip all the way down and the SWR is still higher on channel 40, then the whip is too long. In this case, it is permissible to shorten the whip by cutting the end. If the whip is painted, scrape the paint away after cutting so that there is good electrical contact with the set screw. DO NOT cut more than 1/2 inch at a time. Before cutting, refer to the above (A through C). Your trouble might be an installation problem, not that the whip is too long. Be careful! If you cut too much, you may wind up with an antenna that is now too short.
E. The whip is too short (steel whip antennas). Again, first refer to the above (A through C). Your problem may be installation-related. But it is possible that certain applications may require a longer whip than what was supplied by the manufacturer. If you have raised the whip as high as it can go and the SWR is still higher on channel 1, contact the antenna manufacturer.