The author is an amateur radio operator and has used the method described in this article to tune several mobile radio installations.
Items You'll Need to Set SWR
You'll need the items listed below to set the SWR of your ham, GMRS, or CB antenna and transceiver.
Please note that not all antennas require adjustment, as some may already be properly tuned from the factory.
- SWR meter: You'll need an accurate SWR meter, such as the Surecom SW-33 MK2 or Nissei RS-40, both of which we've used with good success. (See notes below on choosing the correct meter for your radio.) Depending on your model of transceiver, you may need to purchase an antenna connector adapter and short length of cable to connect to your radio's output. Most ham radios and CB's use female Pl-259 connectors on the back of the set.
- Small Allen wrench set: Many amateur radio and CB mobile antennas are adjustable by loosening a small Allen screw that holds them in place.
- Bolt cutter: To cut a stainless steel antenna you'll need a strong bolt cutter. (Be sure to wear eye protection when using one.) You can also use a grinder to take off small lengths of antenna.
Step 1: Check the SWR of Your Radio and Antenna
The first step is to check your radio and antenna's SWR to see if any adjustment is necessary.
- Start by moving your vehicle to an outside area, far from metal objects.
- Connect your SWR meter in line between your transceiver and antenna.
- Set your radio's transmit power level to "low."
- Tune the transceiver to the middle of the amateur band that you'll be using, or to the CB or GMRS channel that you plan to use.
- Check to see if the frequency is clear, then give your call sign—if applicable—and announce that you'll be testing, such as "W4ABC" testing.
- If your SWR meter requires adjusting, do so before taking a reading. Otherwise, key the microphone and write down the reading. If it is below 1.5:1, you can operate your transceiver safely at that level, and no further adjustment is really necessary. (Lower is always better, but proceed with caution if removing any length of antenna, as this cannot be reversed!)
Step 2: Tune the Antenna
If you have a high SWR reading, the next step will be to tune the antenna so that it's resonant at the frequency range you'll be operating on the most. To do that, you may only have to move the antenna up or down a bit without cutting it.
In some cases, though, you may need to take some length off the antenna using your bolt cutters. We'll get to that later; for now, the first step is to check the SWR reading to know "where you're at."
- Using a black permanent marker or pencil, make a thin line where the stainless steel antenna fits into the base, as seen above.
- Loosen the Allen screw and raise the antenna element upwards, as far as you're able to, then tighten down the screw.
- Check the SWR again. If it's higher than before, the antenna element needs to be shorter to be resonant. If the reading is lower, and you've moved it upwards as far as possible, this is all you can do since you can't add length to the element. A reading below 2.0:1 is generally not harmful to most transceivers, though lower is better.
- If the reading was higher when you raised the element, try lowering it 1/16" below your black mark. Check the SWR reading again. If it's getting lower, you're heading on the right course. Write down the reading and keep lowering the element and repeating this process if necessary.
- If your reading is 1.5:1 or less now, there's no need to proceed any further. While a lower reading is always better, if you begin cutting length off of your antenna and go too far, you'll have ruined it. If your SWR is still above 1.5:1, proceed to the next step.
Step 3: Shorten Your Antenna
If lowering your antenna is reducing SWR, and you've gone as far down as possible and are still above 1.5:1, then shortening the antenna element is the next step.
- Place the antenna element into a vice and secure it tightly.
- With your bolt cutters (don't forget eye protection), carefully remove no more than 1/2 cm or 13/6 of length from it.
- Return the element to your antenna base, lower it all the way down and tighten the Allen screw. Check the SWR again. If the reading is below 1.5:1, you're done; otherwise, repeat this process until you get the reading you want.
Please note that once you've removed length from your antenna it cannot be reversed, so proceed very carefully.
Other Factors That Can Cause High SWR
The following are some other things—besides an antenna being too long or short—that can cause high SWR readings.
- A short in the antenna cable or connector. (A multi-meter can be used to test for this by checking for any shorting between the outer coaxial cable and inner conductor.) There should be no conductivity between your antenna cable's inside and outer part.
- Antenna placement too close to the vehicle's cab or other metal objects can sometimes cause a high SWR reading. Moving the antenna farther away from the cab or other objects may fix this problem.
- A buildup of bugs or road grime may be causing a short between parts of the antenna, which are meant to be insulated from each other. Cleaning the antenna may solve this issue.
Tips for Choosing an SWR Meter
It's crucial that you use an SWR meter that's made for the frequency range of your ham radio, GMRS or CB. CB radio operates in the 27 MHZ range, so you'll need an SWR meter made for that range.
Since we own a GMRS radio and also a dual-band VHF-UHF ham radio, we use the Surecom SW-102, which is made to work from 125-525 MHZ. This is a good SWR meter for this range; however, it's not meant for use with CB radios. Another good "no tuning required" meter for VHF-UHF is the Nissei RS-40, which we've also used with good success. An example of an SWR meter that works for CB radio is the Nissei RS-27.
When choosing an SWR meter, It's best to choose a model that doesn't require calibration or tuning before each use. This will ensure consistent readings as you perform the steps above in tuning your antenna. Also, models that don't require calibration reduce the amount of unnecessary transmission over the airwaves during testing.
Lastly, be sure to purchase the proper connectors that you'll need to hook up the meter to your radio. The Surecom SW-102 is made with SMA connectors. To use this meter on most mobile transceivers, you'll need a pair of SMA female to UHF SO-239 adapters, as well as a short cable to connect the meter to your radio.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.