Tuning Your CB Radio Antenna
The most important and often overlooked step in installing a mobile CB is tuning the antenna. While a properly tuned antenna will result in strong radio performance, a badly tuned or untuned antenna can result in poor performance and can even damage your radio.
Before you start fiddling with the antenna, it's important to understand the basic theory behind tuning as well as the term "Standing Wave Ratio" (SWR).
What Is Antenna Tuning?
Tuning is the process of adjusting an antenna so that the radio's output power is turned into radio signals as efficiently as possible. In more tangible terms, it involves incrementally increasing or decreasing the length of the antenna. The goal of antenna tuning is to match the antenna's physical length with that of the radio frequency being used, so as to ensure optimal performance and transmission efficiency.
An antenna's efficiency is measured by the Standing Wave Ratio, or SWR, which represents how much of the radio's power is being turned into radio signals by the antenna. An SWR reading of 1.0 indicates that 100 percent of the radio's power is being transmitted, while a ratio of 3.0 means that only a third of the power is being transmitted by the antenna.
The goal of antenna tuning is to achieve as low an SWR reading as possible. While low SWR readings result in the best performance, high SWR readings have more worrying implications than poor transmission range. In fact, operating a radio at SWR levels above 3.0 can cause damage to the radio because all that excess output power is reflected back at the radio as heat, which can result in permanent damage to the unit.
CB Antenna Tuning Instructions
Enough theory - let's tune that antenna! To tune a CB antenna, you'll need to have correctly installed both your CB radio and your CB antenna. You'll also need an SWR meter and a coaxial jumper cable; both can be purchased for around $20 from most CB shops. Now for the step-by-step instructions.
1. Find an open area. Drive your vehicle to an open parking lot or field; make sure there are no trees, buildings or other obstructions within 15 meters. To ensure accurate SWR results, make sure everyone is in the vehicle and that the doors are closed when you take the readings.
2. Hook up the SWR meter. Disconnect the cable going to the antenna from your radio and attach it to the connection marked "ANT" or "Antenna" on the meter. Next take the jumper cable and attach one end to the radio and the other end to the connection marked "TX" or "Radio" on the meter. The SWR meter is now installed in-line between the radio and antenna.
3. Take SWR readings. You're now ready to take SWR readings. You'll be taking readings using both Channel 1 and Channel 40, with the goal of reaching the same SWR level - below 2.0 - on both.
Flip the switch on the SWR meter to the "FWD" setting.Switch the radio to Channel 1.
Hold down the Talk button on the microphone (i.e., "key the mike").While continuing to hold down the Talk button, turn the meter dial labeled "Set" or "Adjust" until the meter reading reaches the position marked "Set."
While continuing to hold down the Talk button, flip the switch on the meter to the "REF" or "SWR" position and write down the numerical SWR reading you see. You can now release the Talk button.
Change the radio to Channel 40 and, repeating steps above, take a second SWR reading.
Depending on your results, you'll have to do one of the following.
Channel 1 SWR is greater than Channel 40 SWR. If the reading on Channel 1 is greater than the reading on Channel 40, your antenna is too short and you'll need to lengthen it. Most fiberglass antennas have a tuning screw at the tip of the antenna that you can loosen or tighten to adjust the length of your antenna. Nearly all other antennas allow you to slide a steel whip in and out of the base to adjust the antenna's length. In either case, make very small adjustments and then recheck the SWR readings on both channels. If your antenna has a weather cap that covers the tuning screw, make sure it's back in place before taking your SWR readings.
Channel 40 SWR is greater than Channel 1 SWR. If the reading on Channel 1 is less than the reading on Channel 40, your antenna is too long and needs to be shortened. Use the appropriate tuning feature, discussed above, to make the adjustment. (In the unlikely - and unfortunate - event that your antenna isn't easily tunable, you'll need to physically shorten the antenna by cutting it.) Make very small adjustments and then re-check readings on both channels. If your antenna has a weather cap that covers the tuning screw, make sure it's in back place before taking your SWR readings.
The goals of antenna tuning are (1) to achieve SWR readings of 2.0 or less on both Channel 1 and Channel 40 and (2) to match SWR levels on the two channels as closely as possible.
SWR below 2.0. You can expect good performance and safe radio operation with SWR levels below 2.0 on both Channels 1 and 40. If the SWR exceeds 3.0 on either channel, the radio should not be used until the high levels are fixed.
- Identical SWR on Channels 1 and 40. Identical SWR levels on both Channels 1 and 40 ensure that you have the most efficiently tuned antenna for use across the entire 40-channel spectrum. However, it's often difficult to get these two readings to match exactly. As long as both channels' SWR readings are below 2.0, the radio is safe to operate and should transmit efficiently.
Continue to lengthen or shorten your antenna based on your SWR readings until you've achieved one or both of the stated goals. When you're done, remove the SWR meter and the jumper cable from your installation and reattach the antenna directly to the radio using the original coaxial cable.
Several problems can keep you from reaching the tuning goals. A full discussion of these problems is beyond the scope of this article, but Right Channel Radios CB Radio Resources Library has several troubleshooting articles that can help you solve common tuning problems.
If you've properly tuned your antenna, congratulations! While other CB owners grow frustrated with poor radio performance, you'll be enjoying the superior operation and transmission range that only a properly tuned antenna can provide.
|Andrew Youderian is the founder and manager of Right Channel CB Radios, an online store specializing in CB equipment for vehicles. Right Channel CB Radios carries a large selection of CB radios and antennas, and hosts a CB Radio Resources Library, containing numerous CB-related articles and guides. Youderian lives in Salt Lake City, Utah, and has traveled extensively throughout the American West. His most recent excursion was a 10,000-mile, five-week road trip across the United States.|