2. What is SWR?

Tech-Talk
Part 1

Hi friends,

As promised, here's the first installment in a series of discussions on SWR, antennas, feedlines, and related topics.

My aim is to give you some solid information based on facts, and to dispel some of the myths, old wives' tales, and just plain bunkum I hear regularly on the air and at Hamfests.  No heavy-duty theory, and I'll keep the math to an absolute minimum.  Some of the topics we'll explore include:

What is SWR?
What does an antenna tuner do?
My antenna easily tunes to a 1:1 SWR, but no one hears me
My SWR is high, so I have a bad antenna
My SWR is low, so I have a good antenna
What's the best feedline to use?
Why does my SWR change when I increase/decrease power?
What does a balun do (and how the heck do you pronounce "balun")?

I strongly encourage you to get the latest ARRL Antenna Book.  It's chock-full of useful information.  True, some of that information is heavy-duty theory and impenetrable math, but there's plenty in there that anyone can use.  And a big plus is that it also includes a CD with (among other things) two very useful programs.  EZ-NEC antenna modeling software, and TLW (Transmission Lines for Windows) can save you hours or days of "cut, try, measure, repeat".  Later on in this series I'll present some eye-opening material using both programs.

Full disclosure -- I have no formal training in Antenna Theory, RF/Electrical Engineering, or any related field.  The material and ideas I'll present come from recognized experts, my own experiments and experience, and plain old common sense.  In case of a major error, I welcome correction.  Nitpicking, whining, carping, and the like will be ignored.  I don't generally have time to answer individual questions on particular installations, but if I feel a question has wide application, I may address it.  So here we go...

The definition of SWR (Standing Wave Ratio) is the ratio of forward voltage to reflected voltage at a particular point on an antenna feedline.  Keep "at a particular point" in mind... it's important.

OK, so where do the voltages come from?  When you key your transmitter, it sends a burst of energy up the feedline at a certain voltage and current.  The antenna at the other end of the feedline radiates some percentage of that energy out as a radio signal.  That's what we want!  And of course, the higher the percentage, the stronger our signal.  But here's the catch.  All of that energy burst radiates only when the characteristic impedance of the load matches that of the source (ignoring the generally small resistive losses in the wires themselves).  Note that "load" and "source" can refer to any point in the signal path.   In a typical Amateur Radio Station, the transmitter is designed to work into a 50 Ohm load.  So we often use 50 ohm coaxial cable, and feed an antenna that is also 50 Ohms.  By definition, we have a 1:1 SWR and everybody's happy.

But what happens when the antenna is not 50 Ohms?  Now some of our transmitted power is reflected back down the feedline instead of radiating out into space.  That reflected power has a voltage and a current value, just like the forward power does.  And the ratio of the forward voltage to the reflected voltage at a certain point on the line is the SWR at that point.

The voltages vary along the line as sine waves.  Since they are out of phase (they don't cross zero volts at the same point on the line) the ratio of the two voltages changes all along the line -- that is, the SWR varies along the line.  It follows, then, that we should be able to find an acceptable SWR at some point on the line.  But that's a cumbersome process, and is good -- at best -- on only one band.  Enter the antenna tuner!  We'll talk about tuners, and their advantages and limitations, in our next installment.  Until then,

73 for now,
John Bee, N1GNV