This is part two of the two-part blog on Standing Wave Ratio. The second half of this topic is about the devices used to take SWR readings and tips on taking those readings.
Cover me. I’m going in.
So, to pick up where we left off, Bat Fans.
A typical radio setup involves a transmitter, hooked to a feedline (coax), and connected to an antenna. When you transmit, your receiver generates a radio frequency current that travels down the feedline to the antenna in what is knowns as a Forward Wave. The antenna converts the RF current into an electromagnetic wave and transmits that signal outward. But sometimes (okay, almost all of the time) part of the power is reflected back towards the antenna in what is known as a Reverse Wave.
Visualized in terms of water waves, think of a forward wave crashing into a cliff that doubles back on itself. The Reverse Wave distorts the shape of the Forward Wave and creates a Standing Wave, or, what I like to call it -- “Electronic Turbulence”.
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.
Knowing your system’s SWR can tell you if your antenna is too long or too short. Unstable SWR may indicate a problem in the feedline or the antenna.
And to make life even more interesting, as someone brought up last week…SWR can change over time. Even if your system has a great SWR, take a periodic reading. Radios may function differently as they get older, coax shielding can degrade, rainwater may get into the feedline, antennas may move and connections loosen.
When it comes to taking an SWR, it’s important to remember a few things.
When taking SWR readings for mobile or portable antennas, take the readings AWAY from large conductive objects -- this could include anything from light poles to metal buildings to metal rain gutters. Directional antennas should be pointed straight up with their reflectors as far off the ground as possible.
Also, if you have a very long, lossy run of coax, it’s entirely possible to have a decent SWR at the transmitter and actually have no power going out at the antenna. This is due to the cable loss over a long run. If your coax loses 4.5db, it’s possible that, over a 2-300’ run, the wave could just die out within the cable. No reflection back means no standing wave, so the ratio would be 1:1. If the signal dies within the cable, you will get an SWR of 1:1 at the transmitter. Moral of this story, if you have long runs take your reading at the antenna.
There are two different instruments typically used to take an SWR reading. The SWR Meter and the antenna analyzer.
An SWR meter connects to the transmitter and compares the feed line voltage in the forward and reflected directions. To do this, you must transmit in order to take a reading and the reading must be taken during transmission. Multiple readings along a band equal multiple transmissions. Also, SWR meters are usually combined with a wattmeter. Buy an SWR Meter that matches your system, both in power level and frequency. If you’re running a 50 watt uhf/vhf radio, buy a meter designed for a 50 watt/uhf/vhf radio. They range in price from $40 - $300.
The antenna analyzer, however, connects in place of the transmitter. It generates its own signals for the antenna and generates a SWR reading, which is displayed by the frequency number. Usually you dial through a range of frequencies while observing the readout. An analyzer can range in price anywhere from to $250 - $1100.
Due to the difference in price, your first SWR measuring device is probably going to be an SWR meter. So let’s look at that measuring technique. You will need an SWR Meter (duh!) and a short jumper coax with the correct connectors attached, usually PL259.
• Connect one end of the jumper to the back of the radio and the other end to the transmitter socket on the meter.
• Always use minimum power for transmitting so as not to interfere with nearby stations. (Remember, the same FCC rules apply here! If you transmit, identify!)
• All units are slightly different so follow the manufacturer’s directions. Some models may require calibration before each reading.
• Take a reading at the lower end of the band.
• Take a reading at the upper end of the band.
Remember the goals for tuning!!
• Make it usable across the band or bands.
• Tune so that you have equal SWR readings on each end of the band.
• Don’t start adjusting your antenna until you are *sure* that you have enough ground plane for your band. Remember, the rule of thumb for ground plane is approximately one wave length.
• When tuning a new antenna, mark the manufacturer’s original setting with a sharpie before you start adjusting. The antenna will most likely come with an Allen wrench to loosen the top whip part of the antenna. Longer lowers the tuning frequency, shorter raises the tuning frequency.
• Adjust your antenna,
If the high edge of the band reads a lower SWR, then the antenna is too short.
If the low edge of the band reads a lower SWR, then the antenna is too long.
Remember, the 1:1 SWR doesn’t exist, it’s the mythical pink elephant. I saw one once in the Dumbo movie, but even then, it was an illusion brought on by alcohol. Avoid the SWR frenzy. You’re measuring the entire band, not just one spot on the band. If the SWR is less than 2:1 on the entire band, leave it alone. Anything over 3:1 is dangerous.
If you’re a math geek, you might enjoy plotting an SWR curve to find the resonant frequency on the band. Take a reading every few thousand kilohertz and you will find that one particular frequency is the closest to 1:1. Note: It’s not usually in the middle. I say this knowing full well this will trip someone’s trigger and they may actually do it. What can I say? What’s fun for one isn’t always fun for all. If you do the curve, you can also do the math and get an SWR for your entire band, which is called an SWR Bandwidth.
If you’re smart enough to do that, I want you to come teach the rest of us how to do it. MY head hurts.
One last random note about systems. Last week Jay, WB5UDA, mentioned that all of the different ham bands are harmonics of the other ham bands. This was originally a plan set up by the FCC so any interference the ham bands created would be our own. But I read an interesting little tidbit regarding length of coax. It said “It’s best to use coax in multiples of quarter, half, or full wave length of the center frequency of the antenna you are connecting to. Of course there was a formula that went with that, too. I hate formulas. Throw it in the Googlator if your curious. In the meantime...
But I’m growing to … Tolerate … Stand Wave Ratio(s). I may even have to go buy me a new storm antenna just so I can tune it. Bet’cha this time I'll even hang onto the directions!