This is part one of what, APPARENTLY is going to be a two-part blog. In other words, I got sucked into this black hole topic and it’s going to take some finagling to climb out of the SWR topic abyss. Cover me. I’m going in.
This week’s topic is Standing Wave Ratio (SWR). Specifically we’re going to define it and discuss things that can affect it. Next week we will cover the instruments used to read SWR and how to take SWR readings.
Okay, here’s a story for you.
I ventured onto the airwaves in April of 2015 with an HT radio, a mobile magmount, and a Comet SBB-5 dual-band antenna. By that time I got on the air, I was ready to get the show on the road. I had ordered, and subsequently returned and reordered the Comet because I wasn’t paying attention to the connectors involved.
For those of you who do not know me, my middle name is Allison, not “Patience”. So when I received my antenna, I took it out of the package, screwed it all together, slapped the magmount on top of my truck, ran the miles (and miles, and OMG! MILES) of coax through the window and considered it a done deal! No manuals or instructions books for this girl. No sir! Let’s roll!
As a new ham, I had no frame of reference as to what was normal in regards to signal propagation, but at some point I realized that, while my dual band antenna worked well on the 2 meter band, I was having some major issues hitting the 440 band. The “dual” part of the antenna was not living up to the package advertising. Perhaps I missed something while I was wadding up the packaging and aiming for the trashcan?
I mentioned it to the Elmer who immediately said, “So, how did you tune the antenna?” To which I replied, “Uuuuuuuuuum? Tune?” Yeah, apparently, you’re supposed to check the SWR on an antenna and then shorten or lengthen it in order to get the best signal propagation for the band or bands you are operating on.
Who knew? Well, apparently I should have since, you know… I passed the Technician’s test and all that. But lesson learned. SWR is a key component to a good ham system. Unfortunately, the subject of SWR can get really deep really quickly. Case in point: I printed off about six articles on SWR for my research on this topic. The article I printed from Wikipedia went straight into the trashcan immediately after I read it. (That's Ms. Impatient to you, Buddy!) God save us all from bored, over-educated, mathematicians and engineers!
Here’s another newsflash. I’m also not a mathematician or an engineer. In fact, the minute numbers and formulas start getting tossed about in ham radio topics, I do a pretty good impression of Linda Blair in the Exorcist.
I need it simple and that’s the way I’m going to try and feed it to you.
Let’s start with a typical radio setup. You have a transmitter, which is connected to feedline (i.e. coax), which is then connected to your antenna. When you hit the Push-to-Talk button, your receiver generates a radio frequency current (voltage) that travels down the feedline to the antenna. This is called a Forward Wave. The amplitude, or height, of the wave is proportional to the power the transmitter is producing. The bigger the wave, the more power involved. In a perfect world, all of the energy is transmitted out from the antenna.
But as we all know, nothing about life is perfect. Sometimes part of the energy traveling in the forward wave is reflected back in the opposite direction back down the feedline and back to the transmitter. And this is called a Reverse Wave.
Think of in terms of water. As an ocean wave heads towards a shoreline, it’s a perfectly formed forward wave. If that same forward wave crashes into a cliff, however, it reverses direction even as the next forward waves are heading toward the cliff. The friction created distorts the forward wave’s form as the reverse wave hits it and thus creates what is called a Standing Wave. Think of it as electronic turbulence: two waves competing in two different directions simultaneously.
SWR stands for Standing Wave Ratio, or sometimes Voltage Standing Wave Ratio. It is a measurement of how much power is being output versus how much power is being reflected back towards your transmitter or, to paraphrase – it measures the electronic turbulence in your system. In other words, just how distorted is your wave?
The perfect SWR ratio is 1:1 and, for the record, it’s as mythical as a pink elephant, which was an illusion even in the Dumbo movie. But it makes life a lot easier to pretend 1:1 does exists so you can look at it theoretically. A perfect 1:1 SWR has no standing wave because there is no reverse friction to distort the wave. Easy enough, right? Of course not!
There are so many, many thing that can affect SWR, I’m not actually sure they will fit on one page. But let’s list a few and see where we go from there.
1.) Mismatched impedance – matching the impedance of the coax to the antenna. Using 50 Ohm coax with a 50 Ohm antenna, for instance. If you used 100 Ohm coax and a 50 Ohm antenna, you will create a 2:1 SWR ratio by default. **Note this is another one of those theoretical situations / pink elephant things because ALL coax has loss.
2.) Coax loss – when a manufacturer quotes a loss of 3.5 decibels, that is under perfect conditions – i.e. the pink elephant 1:1 SWR. Also, that 3.5 decibel loss is per one-way reflection. In other words, it’s 7 decibels per round trip. Under perfect conditions! Also, also, the manufacturer’s loss quote is not only under 1:1 SWR, but it is specific to the band involved. i.e. 4.5 db loss at 2 meter frequency.
Things that affect the coax loss include
a. Conductor size
b. Length of line between the conductors
c. Type of plastic used in the insulation
d. Right angle bends in coax or excessive coiling
3.) Connectors -- Every connector added to your system adds loss. If, in the case of a magmount antenna, you are using a jumper cable or a “pig tail”, not only have you added an extra connector to your system, but usually the coax used in the jumper is less than desirable coax. Also, if the connector requires soldering, the quality of conductance is in direct correlation to the soldering job.
4.) The location of your antenna is not the best – but sometimes you’re stuck with what you got! Beware of large objects near your system, particularly metal or other conductive objects. (Think buildings, light poles, power lines, etc.)
5.) Insufficient ground plane – antennas require an electrically conductive surface to reflect radio waves, be it the earth or a metal surface. Providing sufficient ground plane by default will decreases influence by external noise because there is less “space” for the noise to infiltrate. The amount of ground plane required will vary based on the band you are operating on, but the rule of thumb is one wavelength.
6.) The antenna is no properly grounded. The base of the antenna must be in contact with something conductive. In the case of mobile antennas, be sure that your antenna is screwed in properly. Having an unsecured antenna flapping in the breeze is basically the same as
7.) Antenna is not properly tuned – SWR reading should be taken at the beginning, middle and end frequencies of the band in question then the antenna should be lengthened or shortened accordingly.
Well, going back to that pink elephant thing. Perfect SWR’s do not exist and the readings themselves can be misleading. Which is next week’s topic. But most modern day transmitters have safety precautions to prohibit radio damage due to bad SWR and reflective current. If there’s too much reflection, the transmitter will simply reduce the power output. However, that’s not a given. Knowing your system’s SWR in paramount to your success and safety.
~73
Allison
KG5BHY