Drivers' Corner - Layover CB Shop

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If you expect to get the maximum performance out of your CB radio, you need to make sure all of your equipment is in good working order. The biggest area that most people overlook is the SWR (standing wave ratio). It isn't as complicated as it seems to do a quick check to make sure everything is in order.

The full, technical explanation of what SWR is and how to check it can be found at The American Radio Relay League website www.arrl.org. Most people not only don't look it up but don't really care about antenna theory-they just want their radios to work! Before I start getting emails and hearing from all the HAMS out there that might feel offended, please don't. This will be a simple explanation for a large number of people that might not have to opportunity to research the subject in depth.

The not so complicated explanation is that SWR is a mathematical formula that expresses how efficiently your antenna system is working. There is no magic formula or secret to an optimum SWR. If you give the antenna a 4-watt signal and all 4-watts radiate off the antenna, the SWR is a 1 to 1.0, this would be considered 100 percent efficient. Since this is not a perfect world, this result is not achievable. An SWR of 1 to 1.5 would be 95 percent efficient and a 1 to 3 would be where about only 50 percent of the signal radiates off of the antenna.

In other words, an antenna is anything that sticks up in the sky that transmits and receives radio waves. The best antenna is a 102-inch steel whip sitting on a 5-inch spring. All other antennas are just variations of this basic combination. Since 107 inches is a bit tall for mounting on the side of a rig, most antennas have been condensed into a shorter antenna with a "center load." Most of these are configured in a familiar K40 Trucker or Wilson 2000/5000 design. Whether tall or short, they are basically all the same.

Now why is this information important? Consider that the more efficiently an antenna is working, the better a radio sounds. Knowing what your SWR is becomes very important because the power that doesn't radiate off of the antenna has to go somewhere. Where does it go? Back into the radio and into the transistor called the final. The final takes electrical energy and changes it to radio waves called RF. The power coming back is called reflected power and this type of power can damage the final. That's when folks like me make money! Drivers don't understand what SWR is and why it's important, because of that we repair a great number of radios. When the transistor goes, it actually explodes and most shops will say you popped a final. Don't throw away the radio. Replacing the final should cost less than $40. Replacing the final and retuning the radio can be done in just about any shop. Protect your radio-have your SWR checked.

First of all you must make certain that your studs are clean. By this I mean not full of corrosion. White deposits collect as water evaporates from the stud. It collects between the stud and the mount. Just remove the stud, clean it with some steel wool and reassemble it. It is a good practice to replace your studs every 6-8 months. Since bending in the wind will eventually pull them apart. Clean the mount with an emery cloth. This is important to getting a good ground for your antenna.

Lastly, inspect your coax to make sure it is not showing signs of degradation as well. Whether it is single or dual is not important at this stage. Make sure there are no visible cracks or cuts. Check carefully where the coax connects to the connector on the bottom of the stud. Look for signs of corrosion or wear. If the coax looks questionable you might be better off replacing it and not taking chances.

Now that you have cleaned and checked the physical aspects of your coax and studs we can get down to checking the SWR. Remember that your antennas are tuned to the truck, not the radio and the radio is not tuned to the antennas either. Your SWR is what it is and must be changed in the antenna system-meaning the antenna, stud, coax and mount.

Let us assume that at this point your stud and coax is in good condition and we can actually test the SWR. One step further in this assumption is that you have a radio with a built in SWR meter (a Cobra 29 series radio is a good platform to use for explanation of checking your SWR). You will want to check your SWR on three different channels to ascertain the current reading. You will need to place the SWR/RF switch in the CAL and key the mic to check the calibration of the meter. The needle should stop at the "upside down" triangle on the upper extreme right corner of the meter. If the needle is not in this location you need to adjust the SWR CAL knob until the needle is in the correct position.

After you have calibrated your meter, you can move the SWR/RF switch to the SWR position. Change the channel to 1, then 20, and then 40. At each channel you will key the mic and check the SWR for the current reading. If your SWR is high on channel 40 and low on channel 1, then your antenna needs to be longer. For a single antenna, these are the steps you should follow to adjust and set your SWR. Use the proper tool and adjust the antenna length in 1/4-inch increments until it is as low as possible across the bands. Yes, you will have to check the settings on each channel after each adjustment. If, however, your SWR is high on channel 1 and low on channel 40, then you have a different method for correcting your SWR. The first and simplest method is to adjust the length of the antenna to its shortest point. If the SWR is still higher on channel 1, then you begin the hard part. You will need to trim the antenna in 1/4-inch increments. You do this by using a set of bolt cutters to cut the metal stinger. Do not shorten the stinger from the top, always cut from the bottom. Also remember the Carpenter's rule: measure twice and cut once. If you are higher on one end but spend most of your time on channel 19, then be sure that the SWR is as low as possible on channel 20.

If you have dual antennas, you might want to seek help from someone who knows a bit more about the correct approach to make sure both are as close as possible. One final note of caution: There are many good shops in this country and then there are also a few rip-off artists. Choose your personal tech carefully and stay with them. If you don't know where to go or who to see, then feel free to contact me and I may be able to make a few suggestions for you.

This article is provided by Truckstop Electronics, Inc. Stan Wells has been involved with repairing and selling CB radios for over 30 years. He's also involved in product development and testing. For more information visit http://thecbshop.com/ or email Stan at cbshop@layover.com.