Setting your Antenna for best SWR
We recently had a listener request we explain how to adjust a mobile CB antenna for best SWR. First off let’s talk about antenna placement. The best place to put an antenna is generally accepted to be placed where it will receive and transmit best. Fortunately if receive is good transmit should work as well. The common antenna to use on our Jeeps is a single “trucker” style fiberglass antenna. Only the outside of the antenna is fiberglass, there is a wire inside, or wrapped around the length of the whip. Most people place the antenna on the rear of the Jeep. What you should keep in mind that the antenna you see is only half of the antenna! The other half of the antenna is an electrical reflection of the antenna you see. That means that you have to have metal to achieve this reflection.
If you connect your antenna to something that is insulated, either the material is not metal or it is painted then that reflected half of your antenna won’t be there, or won’t be there all the time. This will drastically alter the Standing Wave Ratio (SWR) of your antenna system. A much less common place to put your antenna is on the fender as seen in the picture to your left. This provides a very nice connection and reflective surface to create that half of your antenna’s image. It also makes running the coaxial (coax) cable run shorter. Let’s get this out of the way. Coaxial cable can be any length and should be as short as possible. Coax causes loss of signal in both transmit and receive. The shorter it is the less loss. More power to the antenna for transmit and more signal for receive in your radio. NEVER use the length of your coax to adjust the SWR of your antenna system. If changing the coax length changes your SWR you have an antenna problem.
I haven’t used a CB in a long time, or purchased an antenna, so this information may not be current. The four foot trucker antenna was the go-to antenna for mobile operation, and I think it still is. This antenna is “PRE-TUNED”, which means you can’t adjust it. Well let me take a step back. What is tuning an antenna and why do you do it. Well we’re talking about adjusting your SWR so the why is to get the best SWR! The “tuning” is adjusting the length of the antenna. Why does the length of the antenna make a difference to how well your antenna system works? The length of the antenna is tied directly to the frequency of the radio that you are using! The higher the frequency the shorter the antenna! The lower the frequency the longer the antenna. There is a formula. 468 divided by {Frequency} (Ex. 468 / 27.205 MHz = 17.2090 feet). Wait a minute! I never see 17 feet long CB antennas! You’re right you don’t. That is a half wave antenna length and since the metal in your Jeep reflects HALF of the antenna you only need an antenna that is half of that 17 feet, or 9 feet. Okay, Mister Wizard, I don’t see any 9 feet antennas either, well there was that long stainless steel whip that I saw on the van down by the river…
Antennas can be made to look “electrically” longer than their physical dimensions. If you look closely at the 4 foot fiberglass antennas you’ll see wire wrapped around the fiberglass rod. They are making this antenna 9 feet long, electrically. Some of you may remember seeing antennas with large bases, like the K40, a coil of wire is inside that base, making it 9 feet long, electrically. You’ll see advertisements like 5/8’s wave antenna etc… which just means there is more wire in the coil. 4/8’s in 2/4 or 1/2 wave. So 5/8’s it a tab bit longer. I’ll tell you a secret. A coil of wire is not a good antenna! The best antenna is going to be at least a half wave antenna, or better yet a full wave. If more antenna is more better, that means the van down by the river with the 9 foot long antenna is better than that trucker antenna you have attached to your Jeep. Not that having a 9 foot antenna on your Jeep is very practical, especially when you’re off road. 😀
The do make four foot fiberglass antennas with adjustable tips! Now here’s something you can adjust. You’ll need a SWR meter and some patients. Many CBs come with SWR meter built in. They are not super accurate, but the good news is they don’t have to be. People will worry about having an SWR greater than 1.0 to 1 (1:1) That’s a perfect match! All the goodness is coming in and all the goodness is going out, well with the losses built into using non-superconducting material in the antenna and coax. In the real world you’ll notice little difference (if any) with an SWR up to 1.7:1. SWR greater than 2:1 can damage your transmitter if you transmit for a long time. High SWR means that some of the power (wattage) going out to the antenna is reflected back to your radio. This causes the final transistors to heat up. If they get hot enough the transistors fail. Your radio may still transmit but with much lower power and there may even be weird noises involved on your transmission. Some of you are going “OHHHH! SO THAT’S WHAT HAPPENED!”
If you don’t have an SWR meter on your radio you’ll need to buy one. Great news is they ain’t expensive. Amazon recommends this one. https://amzn.to/316vR63 and it comes with a little coax jumper! Follow the instructions that come with the meter to connect it to your radio and antenna system. After this the checking of the SWR is generally the same for built in SWR meter or external SWR meter. Transmit for a short time and set the meter needle to the calibrate, or CAL. Change the meter to SWR and transmit briefly again, noting the SWR reading. If the SWR is 3:1 or greater you have a problem. Verify that everything is connected correctly. This maybe a sign of bad coax, a break in the coax, or the antenna isn’t grounded well to the body. Remember it has to reflect electrically in the body of your Jeep. You’ll need to find your installation/equipment problem before proceeding. It’s likely you’ll see a SWR of 1.5:1 to 1.7:1. Change your CB to channel 40, calibrate and read SWR. Note the reading. Now change the CB to channel 1, calibrate and read SWR. If you see 1.0:1 to 1.7:1 on both channel 40 and channel 1, you’re done. If not, and you have an adjustable antenna, you can make small 1/8th adjustments and recheck your SWR after each change. If the SWR is higher on channel 40 than channel 1 that means your antenna needs to be shorter.
Antennas are a hell of a subject and very interesting how they work. The two biggest gotchas will be your antenna connection to your vehicle and that coax you’ve had for 20 years and it was working 15 years ago when you last used it. 😀
Got questions? Did I miss something? Put it in the comments below. Chances are you’re not the only one that has the question.