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How to Tune a CB Antenna
Congratulations! If you're reading this, you've probably already installed your CB radio and antenna. Good for you! But wait, there's something else to do that can make or break the success you'll have making those contacts with your radio: you'll need to tune the antenna. Why should I do that, you ask?
Why tune my antenna?
Well, simply put, tuning your antenna is the most important step in the installation of your CB radio system to make sure that it's performing at its best. All CB radios come with only 4 W of power, so you'll need to get your antenna system working as efficiently as possible to maximize your radio's transmission and reception. Poorly tuned antennas can lead to increased interference, dropped signals, and lack of ability to make the best quality contacts, which is, after all, the purpose of installing your rig. If you're convinced of the necessity, move on to tuning the CB radio system.
What is tuning a CB radio?
Tuning a CB radio system is when you adjust an antenna to the correct height so that the radio puts out signals as efficiently as possible. That varies, according to the specific vehicle, the place on the vehicle where you've installed it and the ground plane that it has. First, you measure how the radio is performing currently with an SWR meter, and then you adjust the antenna length or location to improve the signal output.
Tuning the radio means that you're trying to get the lowest SWR reading possible. You do that by increasing or decreasing the length of the antenna (depending on the SWR meter readings) in very small increments of about \0xBC inch, then retesting. By increasing or decreasing the antenna length, you'll be matching the length of the antenna with the radio frequency being used. That will give you the best performance and efficiency of transmission. You can have an excellent radio, but without proper tuning it'll be like having a boat anchor installed in your vehicle (to use CB slang).
What is SWR?
SWR means standing wave ratio. An SWR meter measures how well the radio signal is traveling through the antenna system and into the atmosphere, and thus how far your signal will travel and from how far you can receive signals.
The SWR reading indicates how much of the radio's power is being turned into radio signals by the antenna. If you have an SWR ratio reading of 1.0, that indicates that 100% of the radio's power is being transmitted. A ratio of 3.0 is registered when the antenna is only transmitting a third of the radio's power. That indicates a poorly tuned antenna, which doesn't allow the signal to travel through the antenna system correctly. Instead, the transmit power is reflected back into the radio via the coax cable, causing reduced function. If too much power is reflected back, you risk damaging the internal parts of the radio due increased heat generation. To avoid damage, keep your SWR levels below 2.0; levels of 3.0 or higher could indicate a serious problem.
How can the antenna length be adjusted?
Your antenna manufacturer probably has instructions about how to adjust your antenna. Some CB antennas have a tunable tip, a small screw at the top of the antenna, like the Firestik. To adjust that type of antenna, you turn the screw in or out, easily lengthening or shortening the antenna. For other antennas that don't have a tunable tip, you shorten the antenna by cutting off a very small portion (repeat: about 1/4th inch) of the tip at a time.
How to tune a CB antenna: what you'll need and how to get started
Tuning a CB antenna requires nothing more than your installed radio, antenna and an SWR meter. Start with selecting an appropriate location to do the tuning, then move on to connecting the SWR meter. Then you'll be ready to begin actually tuning the antenna with the step-by-step instructions that follow.
Typical CB radio connections with no SWR meter
Choosing the SWR testing location
You'll want to place your vehicle in an open area while tuning your CB antenna. Tuning the antenna while your vehicle is close to other vehicles, buildings, or even people can give you an incorrect reading. Also, each time you take a reading make sure your doors, hood and windows are closed. If your antenna has a plastic cap on the tip, make sure that's in place each time you take a reading.
How to connect an external SWR meter
Some CB Radios come with a built-in meter, like the Cobra 29 Classic. The owner's manual will include step-by-step directions for the operation of the built-in meter.
If your CB radio doesn't have a built-in SWR meter, you'll need to get one. The popular AUSWR SWR meter is affordable and comes with the coax jumper cable you'll need to connect the meter to your CB radio.
Other SWR meters are available, and some include additional meters. A typical CB radio installation will look similar to the picture above. The picture below shows the same installation with an SWR meter added in line. The instructions further below in this article outline connecting an external SWR meter and tuning a CB antenna.
How to tune the CB antenna
- Turn your CB off and disconnect the antenna coax cable from the back of it.
- Connect the end of your antenna coax to the SWR meter where it indicates "antenna." The connector inputs could be on the back and the labels on the front of the meter.
- Connect the short coax jumper cable coming from the transmitter position on the SWR meter to the back of the CB where you took off the coax cable in step 1.
- Close the hood and doors on your vehicle, and make sure there are no people around the vehicle area.
- Turn on the CB.
- Set the CB to channel 1.
- Set the SWR meter to the FWD position.
- Key the microphone by depressing the talk button and turn knob until the SWR meter indicates the "set" position. Unkey the microphone by letting up on the talk button.
- Flip the SWR meter to the "reflect" position.
- 10. Key the microphone and look at the reading on the SWR meter. The lower the reading, the better. If the meter reading is in the red zone, DO NOT operate the CB. Recheck your connections.
- Turn CB to channel 40. Follow instructions 7-10 again.
How to adjust your antenna by matching the channel 40 bandwidth
- If the reading on channel 1 is higher than the reading on channel 40, you need to lengthen your antenna. Alternately, if the reading on 40 is higher, your antenna is too long.
- If the meter reading on channel 1 is 1.2 and the reading on channel 40 is 2.3, you'll need to shorten the antenna to get better efficiency. You can do that by turning the tuning tip or the adjusting ring, or you can take the antenna mast out and trim it about 1/4 inch, and place it back firmly against the coil. Remember, it's easier to cut a little more off the antenna than to have to add some length (remember the barber who cut your hair too short?)
- If your reading is 2.6 on channel 1 and 1.3 on channel 40, your antenna is too short. The solutions are to add a spring, raise the antenna, or reposition the antenna.
- If any adjustments are needed, they should be made in small increments. Re-check after each adjustment. Be sure to have all components on the antenna when testing, including the tip if there is one.
Once you've completed these steps, you can remove the SWR meter from the line--or you could leave it installed for doing periodic checks of your equipment. If space is limited in your vehicle, detach the meter from your radio, with the security of knowing that it would be easy to hook the meter up again now that you know how.
What if my SWR reading is still high?
There could be a number of causes for a high SWR meter reading, and we recommend reading our resource for additional help with a high SWR.