Drivers' Corner - Layover CB Shop

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I tell the story of a driver who came into the store and said, "I lost my SWRs," saying it as though SWR was a word. I have often discussed why SWR is important. The simplest explanation is that SWR is a mathematical formula that expresses how efficiently your antenna system is working. A long explanation here would be too detailed. So, for those who want large amounts of information, contact me directly for a list of publications.

For all the rest, here it comes. Most drivers will ask to have their antennas tuned, expecting you to go out to the truck and perform some sacred right that will make the antenna system work. There is no such thing as a magic formula or incantation.

First of all, you must make certain that your studs are clean. By this I mean not full of corrosion. This white deposit collects as water evaporates from the stud. It collects between the stud and the mount.

Remove the stud, clean it and reassemble it. It is good practice to replace your studs every six months as leaning into 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 the coax. 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 at the bottom of the stud. Look for signs of corrosion or wear. If the coax looks questionable, see about replacing it. With all this done, we can now tune the antenna to the truck.

What I said is important. You tune the antenna to work with the truck not with the radio. When an antenna is tuned to work most efficiently with the vehicle, you can change the radio and not need to retune the antenna.

For the purpose of this article, we will assume that studs and coax are in good condition. The mount looks shiny and clean. Mirror arms appear to be grounded when the door is closed.

The most common antennas used in big trucks are center loaded--another secret phrase that simply means the bulk of the antenna is wound up in rings. Remember, all antennas represent 107 inches of steel standing up in the sky. Since a whip of this height would hit every bridge, it was necessary to wind them down into a size that will work on your truck.

The most popular brands of center loaded antennas are Wilson 2000/5000 and the K40 Truckers. Any number of people make designs of antennas and sell them under a large number of brand names. They are still just a tall antenna wound down to fit your truck.

Finally, tuning a center loaded antenna. For this purpose we will assume you own a Cobra 20 Series radio with SWR meter built in. Calibrate the meter then test the SWR on three channels: 1 - 20 - 40. Now comes the part that I have to think about every time. If the SWR is high on channel 40 and low on channel 1, use the provided Allen wrench and lengthen the stinger in 1/4 inch increments until it is as low as possible across the bands. Low in the center is preferred (since channel 19 is where you will be spending most of your radio time).

If the SWR is high on channel 1 and lower on channel 40, this is where it gets tough. Get a good set of bolt cutters. Trim a 1/4 inch at a time off of the bottom of the coax. Do not cut the top of the stinger. Only cut from the bottom. Remember the carpenter's rule: measure twice, cut once. If tuning two antennas, disconnect the coax from one antenna. Tune the connect antenna to be as low as possible. Disconnect the tuned antenna and connect the other antenna. Then tune that one to be as low as possible. Then connect the disconnect antenna and your SWR should be even lower.

That is the simplest explanation of how to tune SWR. Just remember that nothing is ever as simple as it seems. So, plan lots of time to do it correctly. If you get into something that seems like a disaster, go to a pro. There is no shame in asking for help. Even I refer to the directions printed on the back of the Wilson package. When all else fails, read the directions.

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.