What Is An SWR, And How Important Is It?
What Is A Balun, And How Important Is That?
Standing Wave Ratio (SWR) can be described in many different ways. One of the simplest ways to look at it, is that in order for energy to be transferred from one place to another, a matched circuit impedance allows the most efficient transfer of energy and that applies to RF energy too! Since we have all agreed to standardize on coax feedline and final amplifier stages with a nominal impedance of 50 ohms, it would simply be best if the antenna that you were feeding with this system be 50 ohms impedance too. Such a perfectly matched antenna system would give you a 50 ohm coax, feeding 50 ohms at your antenna, or 50:50 (or 50/50) and can simply be written as 1:1! But, who said life was simple? Unfortunately a lot of things change antenna impedance, even while remaining resonant on your selected transmitter frequency. Moving the feed point off center from a center feed point will change it, lowering it closer to earth ground can alter it, changing the angle of the antenna’s elements can alter its impedance. Operating off an antenna’s resonant frequency can alter its impedance! SWR can also be considered as the ratio between the maximum voltage (or current) found at any point along the line to the minimum value found at any other point along the line. That is why SWR is sometimes called VSWR (Voltage Standing Wave Ratio). That means that if you hook up your final power amplifier stage (PA) to an antenna with a 10:1 SWR, the PA’s 100 volt output might get reflected back until it sees 1,000 volts! Vacuum tubes will usually tolerate such abuse, but try that with even the latest and greatest silicon versions and you have a roasted/toasted MOSFET (Metal Oxide Surface Field Effect Transistor) that will need to be replaced. These days almost every solid state PA designed has something called a fold over circuit that can detect an SWR past 2:1 and place the PA into a much lower power setting. The good news is that you your MOSFET is saved, the bad news is that you are, at best, transmitting with 1/10th the power you intended to.
Most Hams who sweat the details seem to live or die by their SWR to the exclusion of all else. If you ask them if their dummy load is a great antenna because it produces a 1:1 SWR they look at you, reply “don’t ask me a stupid question”, and walk away. If a dummy load presents a perfect 1:1 SWR why ISN’T IT a good antenna? Surely there’s something else that’s not being taken into account! The problem is that your antenna doesn’t need to be resonant or even capable of radiating RF energy to have a perfect 50 ohm impedance with a perfect 1:1 SWR – sometimes a 1:1 SWR alone can be far from perfect!
What is impedance anyway? Well the short answer is that it is the complex sum of a circuit’s resistance and reactance. If you want more details pick up a copy of an earlier part of this ongoing series (Part One).
As a very smart Electrical Engineer/Antenna Designer often says: “first resonate, THEN impedance match”. You want an antenna that is capable of radiating RF into the Aether in the first place, then you need to worry about getting the most efficient transfer of RF energy to the antenna with a properly matched impedance. That’s why a dummy load makes a lousy antenna – it may have a great impedance (50 ohms of pure resistance), but it is just a resistor attached to some form of a heat sink. It transfers all your RF wonderfully, but it all gets turned into heat – little to no RF escapes into the world!
So, let’s say you have the ultimate resonant antenna up there high on you 200 foot tower (a Ham can dream, can’t he?). Maybe we should take a more likely antenna – a folded half wave dipole, which has a nominal impedance of 300 ohms at resonance. That’s a 300:50 SWR or simply 6:1! That’s 33% reflected power! Theoretically, if you don’t burn out your final amplifier with all that power it should get reflected back to the antenna, but since most of us use coax cable each trip across a length of coax cable causes an additional power loss going up in heat! This is one purpose for a Balun (or an Unun) – to match impedances. In a perfect world you might take a 6:1 matching balun and your good to go! Antenna tuners perform the same impedance matching function and often are used in combination with a balun. Placing all the impedance matching hardware as close to the antenna’s feedpoint as possible decreases the cable loss of an antenna with an impedance different other than 50 ohms. A less adjustable method is to place an impedance matching circuit at the feedline of the antenna – either as a simple or a complicated series of LC circuits, or as an open or shorted extra piece of coaxial cable operating as a stub match, or any of the less obvious mechanical antenna match designs that go by such obscure names as “gamma match” or “delta match” (to name just a few). Perhaps later in this series, we shall “dive” more deeply into the subject of how all these types of impedance matching devices actually work!
How is your percentage of reflected power related to its SWR? The equation is:
Just in case you don’t care for mathematical equations, here’s a little “cheat sheet” table that gives the important values in “black and white”. No need to memorize anything, this is primarily here to give you an idea of what different SWR values mean in terms of reflected power, etc.
|Standing Wave Ratio||Power Reflected (%)||Power Transmitted (%)||Return Loss (dB)|
Let me offer a reminder, that just because a percentage of your transmitted power is being reflected, it may not necessarily be “lost”. As long as the increased power your final power amplifier stage sees doesn’t burn it out, this reflected power will get reflected back out to your antenna! Part of it will be lost to your coax cable’s power loss at that frequency and part of it will again get reflected back down your transmission line due to its high SWR.
Balun stands for balanced-to-unbalanced, Unun stands for unbalanced-to-unbalanced. In addition to being used for impedance matching or impedance transforming, it is meant to match a balanced antenna (like a dipole, or a Yagi) to an unbalanced transmission line (like a coax cable), or an unbalanced antenna (like a vertical) to a balanced transmission line (like open wire). One very useful type of balun is a so-called current balun (as opposed to the voltage balun described earlier to transform impedance), otherwise known as a current choke or more simply as an RF choke. We all want all of our transmitted power to go up the transmission line and out the antenna. The transmitted power is located in the center wire and the inner surface of the outer shielding (this is because high frequency alternating current travels on the surface of conductors, not throughout their cross sectional volume. What often happens is that some of the transmitted power is reflected back down the outer surface of the coax, so-called common mode (as opposed as the differential mode that we all want).
You can go cheap with an RF choke that’s as simple as six to eight loops of coax coiled together on a cylindrical form, or a series of six ferrite chokes attaches end–to-end and then covered with some shrink tubing, all the way up to some very nice heavy duty toroid based current balun designs. The closer you place this to the antenna’s initial feed point usually the better.
So, do YOU need a balun? What kind? Well, it depends what you need it for – to connect a balances circuit to an unbalanced circuit? Do you need to transform a particular antenna’s impedance to your transmission line’s impedance (does this call for a 4:1 or 9:1 balun)? Or, maybe it’s just that you need to stop some RF from reflecting down the transmission line and getting into your shack. Your needs will define what kind of balun is best for you!
Is your SWR the only thing to be worried about? Nope, first resonate THEN impedance match! Smarter people than me live by this principle.
Capisce? Perhaps next time we’ll get back to another antenna variety? Anyone out there have any kinds of antennas that are of any particular interest? For that matter, is there anybody out there at all?
-The Editor- (I can be contacted at [email protected])