How to Check Standing Wave Ratio (SWR) on a CB Radio

Goals

This HUB will discuss SWR, or Standing Wave Ratio. I have several goals for this hub.

  1. Explain what SWR is, and why it is important.
  2. Explain how to measure SWR.
  3. Explain what to do to remedy high SWR readings, and what acceptable readings are.

What is SWR?

SWR, properly referred to as "standing wave ratio", is technically the difference in height between the highest and lowest amplitude of a wave in your transmission line. Mismatches in impedance along your transmission line can cause some of your power to actually get reflected back at your radio. Effectively, SWR is a measurement of how efficient your radio, transmission line and antenna are working together. Higher SWR is bad, and means more of your transmitted power is being reflected back to your radio, rather than making it all the way out to the antenna.

What Can Affect SWR?

Standing wave can be affected by many things including but not limited to:

  • Mismatches in impedance along the transmission line. This could be caused by using the wrong type of coax or splicing several pieces of coax together.
  • Cuts, bends or breaks in the coax. If your coaxial cable gets cut along the way, and your shield touches the center conductor, your SWR will read as "infinite", causing the meter to peg out on the far right because you have a short and all of your transmitted power is being reflected.
  • Antenna length and material. Radio waves travel differently through different materials, and generally speaking, lower frequency transmissions need larger antennas. That's why CB radio antennas are much larger than the VHF antennas you see on police cars. A common method of adjusting SWR on mobile antennas is to shorten or length the antenna either by cutting it, or by adjusting tuning knobs.
  • Coax length and material. CB radios are designed to operate with a coax/antenna impedance of 50 ohms. Using other types of cable could result not only in degraded performance, but irreparable damage to your radio.

Cobra 148 GTL SRF/SIG/SWR Meter

Cobra 148 GTL SRF/SIG/SWR Meter
Cobra 148 GTL SRF/SIG/SWR Meter | Source

How to Measure SWR with Built In Meter

To measure your SWR you need a SWR meter. Some CB radios come with an SWR meter built in. You can see in the photo on the right, the signal/RF meter in this radio also has a 3rd line labeled "SWR". This section will cover using the built-in meter on your radio, although the steps are almost identical to using an external meter.

Note: Some radios automatically calibrate their meter, and do not have an SWR adjustment knob on them. If this applies to your radio, you can skip steps 4, 5, and 6.

  1. If you are checking your mobile CB radio, it is best to check the SWR in a wide open area away from buildings, sitting in your vehicle with no people outside the vehicle. As a general rule, you want to check your SWR in the conditions that you will be actually using the radio, because there are a lot of things that can affect it, even including the size of the roof of your car, so eliminate factors that won't be there most of the time during normal operation by moving the vehicle out into an open field or something.
  2. Connect your radio as you intend to use it for normal operation, power it on, and place it on the channel you will be talking on the most.
  3. Locate the "Mic Gain" or "Dynamike" knob on your radio and turn it completely counterclockwise so that when you press the button on your microphone and speak into it, the needle on your radio's meter does not move.
  4. Locate the "SWR" adjustment knob on the faceplate of your radio, and turn it all the way counter clockwise.
  5. Locate the switch labeled "S/RF, CAL, SWR". Place the switch in the "CAL" position. If you are using a newer radio with an LCD screen, press the button labeled "S/RF, CAL, SWR" until you see "CAL" on the screen.
  6. Hold down the button on your microphone, and while holding it, slowly turn the SWR calibration knob clockwise until your meter reaches the "CAL" position marked at the far right side. Do not adjust it past that mark, adjust it just "to" that mark. Release your microphone button.
  7. Locate the switch labeled "S/RF, CAL, SWR". Place the switch in the "SWR" position. If you are using a newer radio with an LCD screen, press the button labeled "S/RF, CAL, SWR" until you see "SWR" on the screen.
  8. Press and hold the button on your microphone. The meter will now indicate your SWR reading.
  9. Turn the "Mic Gain" or "Dynamike" on your radio back to full for normal operation.

Workman SWR Meter (Front)

Workman SWR Meter (Front)
Workman SWR Meter (Front) | Source

Workman SWR Meter (Back)

Workman SWR Meter (Back)
Workman SWR Meter (Back) | Source

How to Measure SWR with External Meter

Another option for checking your SWR is to use an external meter. External SWR meters are handy because you can also use them to verify the accuracy of the meter built into your radio. To verify your SWR with this method, you will also need an extra 3 foot piece (I use 3 feet, some people use longer pieces) of coax antenna cable with PL259 connectors, just like the one you have ran to your antenna. This is because your external meter will basically sit between you and your antenna.

Note: Using an external meter that requires a separate jumper cable WILL change your SWR reading slightly, even if you don't notice it. This is normal, and is not cause for alarm.

  1. If you are checking your mobile CB radio, it is best to check the SWR in a wide open area away from buildings, sitting in your vehicle with no people outside the vehicle. As a general rule, you want to check your SWR in the conditions that you will be actually using the radio, because there are a lot of things that can affect it, even including the size of the roof of your car, so eliminate factors that won't be there most of the time during normal operation by moving the vehicle out into an open field or something.
  2. If you made the small coax jumper cable yourself, verify that there is no contact between the outer shield and the center conductor using a multi-meter or ohm-meter.
  3. Connect the short piece of coax to the "transmit" port on the back of your meter. It might also be labeled "Trans", "Radio", "Xmit", etc.
  4. Connect the other end of the short piece of coax to the back of your CB radio.
  5. Connect the coax coming from your antenna to the "Antenna" or "Ant" port on the back of your meter.
  6. Power on your CB radio, and put it on the channel you will be talking on the most.
  7. Locate the "Mic Gain" or "Dynamike" adjustment knob on the front of your radio, and turn it all the way counter-clockwise so that when you press the button on your microphone and speak into it, the needle on your radio's meter does not move.
  8. Locate the switch on the front of the SWR meter. Place it in the "FWD" or "CAL" position.
  9. Locate the calibration knob on the front of the SWR meter, and turn it all the way counter-clockwise.
  10. Press and hold the button on your microphone, and slowly turn the adjustment knob on the SWR meter clockwise until the needle reaches the "CAL" mark labeled on the far right side of the meter. Do not adjust the needle past that mark, adjust "to" it and no more.
  11. While continuing to hold the button on your microphone, flip the switch on the SWR meter to the "REF", "Read" or "SWR" position. The needle on the meter now indicates your SWR.
  12. Turn the "Mic Gain" or "Dynamike" on your radio back to full for normal operation.

If you want, you can even leave the SWR meter in-line like this while you use your radio. That way you always have a way to perform a quick check. If one day you start to use your radio and the SWR meter needle pegs out, you know right away to stop transmitting because there is a problem.

SWR Readings

SWR
Reflected/Lost Power
Description
1
0%
Perfect
1.5
4%
Very good
2
11%
OK
3
25%
Serious problems
SWR readings and what they mean

What Do I Do With This Information?

Now that you know what your SWR is, it's time to make a decision as to what to do with that information. First and foremost, here's my suggestion. Calibrate your meter and check your SWR on channel 1, then go to channel 40, re-calibrate the meter and check the SWR. If your SWR is at or under 1.5 on both channels 1 and 40, don't touch anything, you can skip this part of my article and come back later if something changes and you need to know what to do. SWR at or below 2 is considered acceptable, however I like to get mine down to at least a 1.5. If your SWR falls at or below 1.5, your setup is operating very good and there's nothing you need to do to it.

SWR changes across the band. If you calibrate and check your SWR on channel 1, then flip to channel 40, chances are the readings will be slightly different. This is normal. If your SWR is higher than a 1.5, you might want to do some antenna tuning. This is accomplished in a variety of ways depending on the antenna. Some antennas have tuning knobs, some have tuning screws, some steel whips can be raised or lowered in their magnetic bases, etc. If your SWR is higher than 2, then you have some noticeable signal loss, but your CB radio will function. If your SWR is higher than 3, then you should stop transmitting except for what is necessary to re-check your SWR. Normally, if your SWR is 3 or higher, it's an indication of a short where the outside shielding of your coax is touching the center conductor. This can also happen if you are using a steel whip antenna and it comes into contact with some un-painted metal on the body of your car.

Note: Some base station antennas, such as the Solarcon/Antron A-99, and some magnet mount antennas with tuning coils like the Wilson 5000, will show a DC short if you check the end of the coax cable with an ohm-meter. This is normal as long as your SWR is also low.

If you want to tune your antenna, I recommend staying away from methods that involve removing material unless absolutely necessary, because once you start taking wire cutters to a steel whip, you can't put that material back on. Once it's gone, it's gone. To find out whether you need to lengthen your antenna or shorten it, you need to calibrate your meter and check your SWR on channel 1 and channel 40. Re-calibrate your meter before taking a reading whenever you change channels, because the calibration point and SWR reading will be different on each channel. If the SWR on channel 40 is higher than the SWR on channel 1, then your antenna needs to be shortened. If the SWR on channel 40 is lower than the SWR on channel 1, then your antenna needs to be lengthened. If you discover your antenna is too short, try moving it to an area where it has a larger metal surface immediately under the base of the antenna. Adding a spring to stud mounted antennas is also a good way to add length to your antenna. Some antennas have built in tuning screws and things. Refer to the manual, or contact the manufacturer of your particular antenna if you are not sure how to go about adjusting its length for your setup.

When it comes to base station antennas, they are USUALLY tuned for the middle of the CB band (channel 20) in the factory and don't need to be bothered, so if you have high SWR on your base setup, I recommend checking your antenna cable for shorts, making sure it doesn't have any sharp bends, and placing your antenna in an area where it has at least 25 feet of open space in all directions. If you really believe that your antenna needs to be tuned, refer to the manual for your particular antenna, or contact the manufacturer or your local CB shop.

If you are in a mobile setup (not just mobile radio, but actually in a car) the first thing I would check is to make sure your coax is not being pinched anywhere. Do not run your antenna cable through a car door. Run it through the firewall or something where it can remain stationary. That means no putting your antenna on your trunk either where it will get bent and flexed every time you open the trunk. Make sure there are no sharp bends or kinks as well, and if you have to coil up excess cable, try to make the loops larger than 12 inches across. In my Ford Taurus I couldn't access the rubber grommet around the wiring harness in the firewall, but I was able to run it under the dash from the radio, through the rubber grommet where the wiring harness for the door comes in, and then forward into the fender and away from the moving parts of the door, then up and out and up the edge of my windshield to the antenna which is top center on the roof of the car. You may also notice that you can change your SWR by simply moving the antenna around. Generally speaking, the highest point on your roof is the best location, but in my pickup I actually have a stud mount on the very rear of the bed rail.

In Conclusion

I hope that this article has at least demonstrated how to measure your SWR, explained what it is, and how to remedy problems that can cause high SWR. If you have any comments or questions feel free to post them here, or you can e-mail me at: marcusdean.adams@gmail.com

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