A Mini Primer on SWR Measurements
By John Breckenridge, WB6FRZ
Acknowledgments: I wish to recognize the thoughtful review of this article by Al Bloom, N1AL, truly one of Sonoma County Radio Amateurs' guiding lights. I also credit M. Walter Maxwell, W2DU, the author of "Reflections, Transmission Lines and Antennas," the book that inspired this article.
I have long noticed that many new-time and old-time amateur radio operators alike hold the idea of low SWR (standing wave ratio) for antenna systems very near and dear to their hearts. High SWR has been blamed as being the cause of TVI, poor transmitted signals, burned finals and antenna system problems. Low SWR has been the most sought-after and trusted measurement among hams for antenna system adjustment and monitoring. So let's consider what SWR measurements are, why we make them in the first place, and how we can best interpret the readings we measure.
What Is SWR and What is It Used For?
SWR stands for standing wave ratio. SWR is computed from the ratio between an RF signal going in the forward direction (toward antenna) and the RF signal going in the reverse, or reflected direction (toward transmitter) on a transmission line. Any discontinuity or impedance mismatch along the line or at the ends of the line will cause a portion (at least) of the RF signal to reverse direction. The SWR meter detects the magnitude of that reflected signal in relation to the magnitude of the forward signal. From that information, certain conclusions are then drawn about the quality of the antenna or antenna system. These conclusions rest solely with YOU, the operator.
You will also hear the term VSWR, which stands for voltage. standing wave ratio. SWR and VSWR are the same thing.
Let's do a little math, very little math, to clarify this ratio.
We know that:Forward Voltage + Reflected Voltage
If Forward Voltage is 20 volts and Reflected Voltage is 0 volts, then we get:20 + 0 20
That 1 is the first number in the ratio. The second number, after the colon (:), is always 1. So, our answer is a VSWR (or SWR) of 1:1, pronounced "one to one."
How is SWR Measured?
Whatever we use to measure SWR must somehow measure the forward and reflected signals and mathematically manipulate the values. The result may be shown directly in SWR on a meter, or as a swept value of return loss or SWR on an analyzer, usually a very expensive piece of test equipment.
The most common and cheapest way to measure SWR is with an SWR bridge (SWR meter). Inside there are diodes and other circuitry that allow a meter needle to move. As you might imagine, these meters do not necessarily give the most accurate readings. However, for all but the purist, these simple and relatively cheap meters do a reasonable job for amateur radio work, although uncertainties are magnified at VHF and above.
What Does a Low or High SWR Mean?
As you read the explanations given here, try not to make any judgements or jump to conclusions about the meanings.
A low SWR refers to a large forward RF signal and a small reflected signal. Since the reflected voltage can never be less than zero, the very lowest value possible is 1, or 1:1.
A high SWR refers to a large reflected signal. For example, a meter reading of 9.5 indicates an SWR of 9.5 to 1.
Incidentally, you may note that there are not many numbers on the meter after about 3 or 5. That's because SWR values get very large very quickly. It is difficult to tell the difference between an SWR of 9.5 and 35, for example.
Low SWR is Good and High SWR is Bad, Right?
Wrong, wrong, wrong. That's like saying cold water is good and warm water is bad. To a penguin, this is probably true. But to a person who fell through a hole in an ice-covered pond, It is false.
Like water temperature preference, the interpretation of good, bad, or indifferent SWR depends upon the situation! Sorry, folks; there just isn't any quick and simple answers here. Ask a few hams and I think you will hear statements like: "A good SWR reading means my antenna is working properly," or "A bad SWR reading means there's a problem with my coax or antenna." It's a shame that these simplistic, well-intentioned, widely-accepted viewpoints are passed on from one ham radio generation to the next.
Now that I have your attention, let's consider some situations of good high SWR and bad low SWR.
Examples of Good High SWR and Bad Low SWR,
Suzie Hamm has been a ham for years. Her old antenna system, fed with 23 year-old coax cable, has never given her any problems! In fact, her SWR meter always shows a very low SWR. "If it ain't broken, don't fix it," I've heard Suzie say. The problem Is that Suzie's coax has become so lossy over time that her antenna could be disconnected and she would still get an SWR reading of 1.5 to 1! But how could this be? Remember, the SWR meter is stupid, It can't interpret what is causing it to read the way it does. The SWR is low because the lossy coax cable absorbs the forward RF energy going to the antenna, and also absorbs any reflected power (if there is any left to be reflected) from the antenna going back toward the transmitter. OF course the SWR is low! The solution to Suzie's situation would be for her to periodically measure the power at each end of her coax cable with a dummy load attached. Without changing the transmitter power, the difference in the readings is the amount of power lost through the coax. Low SWR does not guarantee a better signal.
Baluns are wonderfully magical devices; they make everything okay. In fact, some hams have been able to reduce the SWR of their dipole antennas by simply inserting a 1 to 1 balun at the antenna terminals. The SWR doesn't rise at the band edges like it did when the balun wasn't there! OOPS, a red light should go on here, but normally hams will just be pleasantly surprised at the quality of that balun and brag about it to their buddies. What they should be doing is throwing the balun in the trash and replacing it with one that works. Here's what is happening: The balun was made of a ferrite core that saturated when power was applied. The core material provided significant power loss, turning that precious RF into heat before it even got to the antenna! Low SWR does not guarantee a better signal.
Consider a ground-mounted HF quarter-wave vertical antenna system with 12 radials. Its input resistance is determined by adding its radiation resistance and ground loss resistance. Radiation resistance for this antenna is close to 32 ohms and ground loss resistance is on the order of 20 ohms. 32 plus 20 gets you 52 ohms, and your SWR meter shows a perfect match using 52-ohm coax. So that means an optimum vertical has 12 radials? Nope, because the power "radiated" by that 20 ohms of ground loss resistance is lost warming the worms in the ground! Add 25 more radials to the system and the SWR at resonance will rise because ground losses decrease. The SWR may now show 1.4 to 1, but the worms get colder because your vertical is a more efficient radiator! Low SWR does not guarantee a better signal.
Let's take the above example one step further. Let's say you don't like radials so you decide to install only four of them onto your vertical. Your radiation resistance is still about 32 ohms. But now your ground losses are also on the order of 32 ohms, giving you about 64 ohms for input impedance. That equates to an SWR of about 1.3 to 1 at resonance. The SWR is still "acceptable" to your rig, so you move off frequency and find a wonderful thing! The SWR seems quite "flat" across the band; it doesn't rise as you might expect - It's a miracle! You always suspected that you didn't need all those nasty radials, anyway. Nope, sorry. What has happened is that the ground losses are so high now that they hold the off-resonant SWR to a lower value than would be normal if you had a decent radial system. You are really making the worms uncomfortably warm now because half your power is heating the ground. Low SWR does not guarantee a better signal.
Joe Hamm Is testing a new Yagi beam antenna. It seems like no matter what Joe does to the antenna, the SWR is 3 to 1 or more. Joe tries changing the length of the coax because he read somewhere that doing that tunes the antenna. And guess what, the SWR drops to 1.2 to 1! Obviously Joe has confirmed that the length of the coax tunes the antenna, right? Wrong. Joe had a problem before, and still has a problem, although his SWR meter suggests that he fixed it. The problem is that the RF is squirting out of the end of the coax and flowing on the outside of the coaxial braid. What this does besides allowing the coax to radiate and make for totally bogus SWR readings is seriously affect the front-to-back ratio of Joe's beam. The beam no longer shoots its energy properly in the desired direction. This is not good. To correct all of this, it is very important to have a well-engineered balun just ahead of the beam's gamma matching network. If for some reason the balun is not taking care of the problem, then ferrite beads can be placed over the outside of the coax at the connection to the antenna to "choke off" any stray RF that wants to misbehave. Low SWR does not guarantee a better signal.
Fred Frequency has a multiband HF antenna system and feeds It with 100 feet of Belden 9913 coax cable. The other end of the coax is connected to a tuner, SWR bridge, and then to a transceiver. Fred operates 80 through 10 meters with this setup. Fred adjusts the tuner for an SWR reading that shows a good match to the rig. But wait, you say. For various bands, isn't there still a very high SWR (mismatch) between the output of the tuner and the system? Yes, I will explain why. Yes, it's true that coaxial cable losses increase as the SWR increases. From the ARRL handbook, an SWR of 7 to 1 between the tuner and the line creates 1.3 dB of reflection loss in the line. The ham in contact with Fred will see a difference of a quarter of an S unit due to the TINY losses in Fred's cable from a 7 to 1 SWR. When was the last time you heard the signal strength difference of a quarter of an S unit? High SWR does not mean a worse signal.
Now let's look at situations showing what most of us are used to seeing, good low SWR and bad High SWR.
Examples of Good Low SWR and Bad High SWR
Juan Deebee has a 20-meter station with transceiver, SWR meter, and coax cable to a beam at 145 feet (oh, that I were Juan). Anyway, when installed, his beam showed these SWR readings:Frequency SWR
This Is a reasonable characteristic and is an indication that things are as they should be. But two years after it was installed, the SWR at 14.15 MHz increased to 3.4. This high SWR is bad and is an indication of trouble. Now I'm glad I'm not Juan, because he will probably need to climb that 145 foot tower to find the trouble. Of course, he will check the coaxial connectors that are in the shack first. If he's smart, he will rent a helcopter to work on the beam.
Harry Hamm (Suzie's brother-in-law.by a previous marriage and Joe's older nephew) is installing a tuned 40-meter dipole. His transceiver connects to an SWR meter, then to 75 feet of RG-58U coaxial cable to the center of the dipole. Harry has the following SWR readings:Frequency SWR
Harry has a good setup here. He has no tuner, but Harry adjusted his antenna for good low SWR at the frequency that allows operation over the entire band. The radio matches into he antenna system with no problem. The SWR increases toward the band edges, but with a simple dipole, it should! If it doesn't, suspect trouble.
So, Who Can You Believe?
Last December marks my 19th year as an amateur radio operator. That doesn't mean I know very much; all it means is that I've been licensed for awhile. There is a LOT of misinformation about antennas and antenna systems that continues to be handed down from ham generation to ham generation. You new hams can learn a lot from "old timers." But be mindful that myths have been around for a long time and some of us "old timers" never did get it right. It took me years to find out that reflected power in a low loss transmission line is not wasted, is not dissipated, does NOT come back down the line and burn out the final amplifier. I learned that in a low-loss system, most of that reflected power is transmitted by the antenna. It might have to jump back and forth a bit, but those reflected signals are radiated!
If you don't believe me, it's okay, I understand. With the disinformation I have heard over the years, I could hardly accept some of these ideas myself. We condition ourselves through repetition and constant bombardment of the wrong information until we accept it as "fact"' without even considering its validity based on scientific principles.
An SWR meter can be a handy tool for the average amateur radio operator. But many of us have not learned enough about antennas, feedlines, and reflected signals to determine if a particular SWR reading is good or bad. And then when we really determine that it's bad, we haven't learned how to correct the situation. The point is not "to have a low SWR reading." The point is "to have an efficient antenna system; one that radiates well." And many times it is also important to have a flexible system that covers more than one band.
Finally, I want to offer you a solid basis on which to build your antenna knowledge, as well as dispel some of the stuff you may have believed was true for years. I refer you to further study using the book "Reflections, Transmission Lines and Antennas," by M. Walter Maxwell, W2DU. Please, please get a copy of it, if you are at all interested in how RF acts in a mismatched transmission line. It's all in there. The book is published by the ARRL, it's $20 plus change, and you won't be sorry. And no, I don't get a commission out of recommending It.
Everything Walter says is based upon well-established facts of physics and electrical engineering. But much of it is readable enough so that even thick-skulled dolts like myself can understand. For you brainy ones, there is plenty in there to keep you nterested.
Experiment, keep learning, and have fun. For me, that's part of what amateur radio is all about.
Best Regards, John Breckenridge WB6FRZ