Antenna Farm – Article 4 – May 2, 2022

By Charles KC6UFM

SWR…Why You Should Not Use It…And Why You Should Use It

Hello ECR Family, and welcome to The Antenna Farm. This is your friendly Antenna Farmer Charles, KC6UFM.

Yes, I know…that title may be a bit confusing, but simply put, there are many reasons why you should not use SWR for anything and at least one extremely good reason why you should use it everyday.

SWR is a big subject, so this entire article will be devoted to helping you understand SWR and other closely related ideas a little better. As is my habit, I will introduce the ideas in a simple way that may, in some cases, be an oversimplification. The game plan is as we move deeper into the dirt here at the Farm, we will correct the simplifications and get more precise with the concepts. This article is likely to be the longest in the series as well. Let’s dig in!

SWR stands for “Standing Wave Ratio” and it is, as the name states, a ratio comparing two values, similar to the way a decibel works. In technical terms, there are three types of SWR that are (or have been) of interest: (1) Voltage SWR (or vSWR); (2) Power SWR (or pSWR); and (3) Current SWR (or iSWR). For our use, pSWR and iSWR are almost never used outside of a physics lab. In practice, the only one that matters and the only one we will talk about here is vSWR. In common usage, when you see “SWR”, it can be safely assumed that we are talking about vSWR.

If you would like to calculate vSWR yourself (for some reason…maybe you like headaches) I’ll refer you to the rather good article on SWR on Wikipedia. If, however, you aren’t comfortable with mathematical concepts like imaginary numbers, vector analysis, and other more esoteric constructions, just ignore this entire paragraph.

What vSWR tells you is how well the feed line (coax or open wire) impedance is matched to the load (antenna) impedance. This is important because ONLY when the impedance of the feed line and the antenna are exactly equal will maximum energy be transferred into the load. Any other condition results in power being wasted as heat in the feed line.

Essentially, a particular vSWR value is a comparison between the Forward Voltage (energy sent from the transmitter to the antenna) and the Reflected (or Reverse) Voltage (energy that comes from the antenna toward the transmitter). The forward voltage is easy to understand…this is simply the energy created by your transmitter that is sent toward the antenna via the feed line. The reflected voltage is, however, a little harder to grasp…this is the energy that, when it meets the interface between the feed line and the antenna, literally bounces back down the feed line toward the transmitter. Yes, for the purists, this is a simplification and the actual calculation is more complex, but for our needs, this does the trick.

As you probably have reasoned out, there can be an infinite number of values for the resulting vSWR because there are an infinite number of values for the forward and reflected voltages. In practice, we tend to ignore the fine details in the value of vSWR, especially when the result gets big. And that leads to the First Rule of vSWR Values:

Don’t Sweat The Petty Things (and Don’t Pet The Sweaty Things)

Actually, this is a good rule for just about everything in life.

Remember that we’re talking about RF here, and the value of the forward voltage can vary between some positive maximum value, down to zero, and then swing down to some negative maximum that is (usually) the same numeric value as the positive maximum. For example, let’s say we have a generator (transmitter) able to deliver 1 volt at 1 Hz. If we put a voltmeter across the line, we will see the value the meter reads swing up to +1 volt, slide back down to 0 volts, further fall to -1 volts, and lastly rise back to 0 volts again. This cycle will repeat once a second.

Now, imagine another voltmeter that can read the voltage coming from the antenna back to the transmitter, that is to say, it can read reflected voltage. Connect this new meter to the exact same place as the first meter. The meter reading the forward voltage will behave just like in the paragraph above, and our new reflected voltmeter will also do the same thing, BUT (and here’s the important part) the values shown by the forward and reflected meters will be different, sometimes radically different! So the question is, how and why are they different?

Since the voltage values read by the two meters can have an infinite number of possible values, we will look at three “special cases” to keep things simple…

The first special case is if the impedance of the feed line and the antenna are exactly equal. In this case, all energy sent down the feed line will be absorbed (and used) by the antenna load. In this case, no matter what the value shown on the forward voltmeter is, the reflected voltmeter will always read 0 volts because there is no energy coming back to the transmitter.

The second special case is when the end of the feed line is a dead short. In this case, there is a load, but its impedance is 0 ohms. When the voltage from the transmitter hits this shorted load, the current goes to infinity. (Remember Ohm’s Law? I=E/R? I=1/0, and try to divide anything by zero and the result is undefined.) This causes 100% of the forward voltage to be reflected back at the transmitter.

The third special case is when the end of the feed line is an open circuit, that is to say that the load impedance is infinity. Back to Ohm’s Law and we find that I=1/inf and anything divided by infinity is also infinity. Once again, 100% of the energy is bounced back to the transmitter.

Here’s another important thing to remember…the reflected voltage will be shifted by 180 degrees, just like an image in a mirror. In the case of either a short or an open at the end of the feed line, the reflected voltage is said to be “phase shifted” by 180 degrees so that when the forward voltage is +1 volt, the reflected voltage will be -1 volt. Since the energies are moving in opposite directions in the feed line, the net result is 0 volts making it to your antenna.

Now, all of these special cases make one huge assumption, and that is that there is no reactive component. This is NOT how reality works. Every feed line and every antenna has a certain amount of both capacitance and inductance along with its resistance. These values add reactance into the game in a way that shifts the “pure” reflections to some value more or less than 180 degrees. Yeah, it gets messy.

Because of these odd-angle shifts, you should NEVER use vSWR to evaluate an antenna and feed line system. Reality is much more complex, and you can account for that by using an actual RF voltmeter to measure the forward and reflected voltages, an oscilloscope to measure the phase angles, and plugging the numbers into the appropriate formulas. I figure a cheap Bidirectional RF Voltmeter and a suitable cheap scope won’t set you back more than $1000 or so, and the measurements and calculations shouldn’t take more than an hour. But, by golly, they will be right!

On the other hand, a decent SWR Meter that will cover several bands and read the needed value directly will be well less than $100 and takes maybe 10 seconds to get a reading. Is that value real? Nope. Does it matter? Not even a little bit. This means you can use said SWR Meter to take a reading of your approximate vSWR every time you key your mic.

(Just a little trivia, but there are two ways to calculate the trajectory of a spacecraft: Using everyday Newtonian geometry or using the infinitely more complicated Einsteinian geometry. Einstein was right and the results are FAR more accurate. It just takes a supercomputer a week to make the calculations. Newton is pretty darn close and you can do the math on a pocket calculator in maybe 10 minutes. NASA uses Newton, and they put the Webb Telescope through a hole in space less than 20 meters on a side nearly a million kilometers away.)

“So what?” you ask? Easy…a sudden change in your vSWR is never a good thing. Ever. There are dozens, if not hundreds, of reasons that can cause a shift, but they are all trouble. Using the cheap, inaccurate SWR meter will let you see problems before they fry your rig’s finals.

Modern transmitters are not very tolerant of a lot of energy coming back at them. Most manufacturers say that your vSWR should never be above about 2:1 or so. Handy-Talkies tend to be a little more tolerant because of the horrible antennas they all come with. To protect the transmitters, most modern rigs have “Fold-Back” circuits that sense the vSWR and reduce output power to protect the final amplifier devices. My Yaesu FT-991A completely shuts off the output at anything above 5:1 and at 2:1 maximum output is 10 watts. As an aside, the old vacuum tube rigs can handle high vSWR much better than modern solid state transmitters.

So, the big reason why you should not use SWR for anything is that it just isn’t right and has only marginal meaning. But the simple fact is that SWR is quick, easy, and cheap to measure and can give you an indication that something may be wrong so you can dig deeper into the cause before you fry your radio. I have an old Radio Shack “Realistic” SWR meter I picked up at a local swap meet for $5 and I’ve developed the habit of looking at that every time I key the rig. If the value is unusual, I can immediately unkey.

If you are using a simple SWR Meter, don’t get wrapped up with the value. In practice, anything less than 1.5:1 is good enough. Remember that the meter is not all that great and likely the accuracy won’t let you get much closer than that. Also remember that as you change frequency, even slightly, the vSWR will change. Personally, when using a simple SWR Meter, I almost never even look at the values…I just look for the lowest spot.

And remember that a “Good SWR” has no connection to how well an antenna will work. Just as an example, a good dummy load will present exactly 50 ohms with no reactance to the transmitter, so the vSWR will be a “perfect” 1:1, but that dummy load will not radiate any signal. (True…most dummy loads you can cheaply buy DO radiate…I’m talking about the really good ones. You know…the ones that NASA and the military pay $1-million for.)

But there are better ways that, in terms of both price and performance, fall between the SWR meter and the full blown RF voltmeter/scope…

Antenna Analyzers – There a ton of them on the market, and the prices are all over the place. Some are great (expensive) and some are just OK (lower priced). They all do the same things, basically. The big advantage to these little devices is that they are more or less automatic. You tell them what you want to do, and they do it. In other words, you need not know anything about what you are doing. The downside is that you are, even with the most expensive models, locked into what the maker thinks you need to do. If you are relatively new to Ham Radio and/or have little or no background in math, electronics, physics, etc. AND you don’t want to learn, I would strongly recommend this path.

VNAs – (Vector Network Analyzers) Inside the box, antenna analyzers are really a VNA with some additional computing power to automate them. You can pick up a so-called Nano-VNA for much less than $100 on eBay and it can do everything that a $1500 antenna analyzer can do, but you will need to make a few calculations on your own. I myself use a Nano-VNA and like it MUCH more than any of the dozen or so different antenna analyzers I have owned over the years. Then again, the math and such doesn’t scare me. The bottom line is that a VNA can do everything an antenna analyzer can do, plus it can do hundreds of things that the antenna analyzers CAN’T do, but you have to work at it. If you have at least some experience with math, electronics, physics, etc. OR you want to learn as much as possible about these things, the VNA may be the way to go.

One last thing that is closely related to feed lines, antennas, and vSWR…

Antenna “Tuners”

Right off the bat, there is one thing about antenna tuners that I want you to remember:

There is no such thing.

Despite the old CB’er folklore that claims all you need to do is trim your coax to tune your antenna, there is only ONE way to get the antenna resonant (that is, “tune” it) and that is to adjust the electrical length of the antenna. A resonant antenna will present pure resistance as a load, but it may not be 50 ohms. You will likely need a matching system of some kind to make your feed line and transmitter happy. This is why antenna tuners are not real, but antenna matching networks are.

Yes…when using a simple SWR meter, moving the position of the meter on the feed line relative to the radio and the antenna makes the needle move, but that is an artifact of how the meter works. Try that with an analyzer or VNA and you’ll find the value is the same no matter where you measure it.

What we Hams call “Antenna Tuners” are really matching devices. They can take one impedance on the input and couple that to another impedance on the output in such a way that makes your rig’s finals happy. Some can also connect between balanced and unbalanced systems.

If, for example, we have a transmitter with 50 ohms unbalanced output, an unbalanced feed line with 50 ohms impedence, and an antenna with 12.5 ohms of impedance, without a matching network, the radio will see a vSWR of 4:1, likely too much for the finals to survive for very long. Also, the interfering waves in the coax will lower receive performance.

If we put an “antenna tuner” between the radio and the antenna, we can use that to match the 50 ohm coax/radio to the 12.5 ohm antenna in a way that the rig will see very close to a 50 ohm load.

It’s worth noting here that most of the “tuners” built into radios these days can only handle about a 3:1 mismatch. Anything bigger than that and you will need an external device.

It doesn’t matter if we’re talking about a matching unit at the rig end or remote at the antenna end of the feed line…same results. It should be noted, however, that a remote unit at the antenna feed point will also lower losses on the feed line due to high vSWR.

And as you will learn in a future article, there are as many ways to match the antenna to the feed line as there are to skin cats.

Next time, we’re going to have a look at the tools you will need to be an Antenna Farmer, both hard and soft-ware.

Take Care & 73