November 19, 2023
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Hello ECR Family, and welcome to The Antenna Farm. This is your friendly Antenna Farmer Charles, KC6UFM.
In this article, we’re going to have a look at feed lines and the loads that love them, so let’s get right to it!
Feed Lines
There is little doubt that having an efficient antenna and feed line system is far and away the most cost effective way of improving your signal. Sadly, many Hams, both beginners and old timers, don’t believe this and instead will go for increased power at the transmitter. The general rule of thumb is that adding power for a given increase in signal costs about 5 times more than getting the same gain through antenna and feed line changes. In practical terms, a system that works “OK” can be made to work “Great” by proper attention to the feed line. The QRP enthusiasts prove this every day by working the world on less than 5 watts of RF…they pay close attention to the antenna gain and feed line loss.
Getting your signal to the antenna is one of the most misunderstood things in Ham radio, and I think I know why…people tend to overthink feed lines. It really is simplicity itself. A feed line has the task of getting your signal from the transmitter to the antenna. That’s all it has to do. Those of us with mathematical, physics, and engineering backgrounds will be fascinated by the interactions of frequency, resistance, impedance, and other traits common to feed lines, but for the average Ham, these are details that can actually be safely ignored in most cases. In a practical sense, all you really need to know is that all feed lines have three important properties:
Loss – Sadly, all feed lines have a certain loss that varies by length, frequency, and vSWR on the line. There are other factors, but these are the biggies. Some of this loss is due to the resistance of the conductors, some from the reactive components, and some from the actual construction of the line. As either the length, frequency of the applied signal, or vSWR increases, the loss also increases. This means that a feed line that works great at HF (below 30 MHz) with a low vSWR might waste 90% or more of your signal at UHF with a poor vSWR. The loss of various feed lines also varies based on the design and construction. As a general rule, the larger the size of the feed line, the less loss it will have at a given length and frequency, but there are more details and that means these are just rules of thumb. A handy place to look this up is on the web at: https://kv5r.com/ham-radio/coax-loss-calculator/ where you can easily compare the loss of various common feed lines under various conditions. The calculator allows you to vary individual items to see the effect of possible changes.
Characteristic Impedance – In absolute terms, you can feed any antenna with any feed line. That’s right…if I wanted to, I could feed a ¼ wave ground plane (about 50 ohms unbalanced) from my transmitter (also about 50 ohms unbalanced) using 450 ohm balanced line. No problem, but there are a few hoops to jump through. It’s much easier to feed the above antenna with an “unbalanced” line that is close to the same impedance. Coaxial cable (what most Hams use) is an unbalanced line that is readily available in ~50 or ~75 ohms. It’s also easy to work with, is relatively cheap, holds up well, and gets the job done. As hinted, there is also open wire line, sometimes called “ladder line”, that has a much higher impedance (usually 300, 450, or 600 ohms), is a “balanced” line, and much lower loss (see the above web site). That whole balanced and unbalanced thing? Don’t worry about it right now…that’s coming in another article.
Velocity Factor – This an expression of the speed at which signals move through the conductors of the feed line and is given as either a percent (like 80%) or a decimal (0.80). A VF of 1.0 or 100% means the signal travels through the line at the same speed as in a vacuum, and that is the speed of light (roughly 300,000 kilometers per second or 186,000 miles per second). Knowing the VF of a feed line is really only important when designing lengths of feed line to use as matching transformers or phasing harnesses. Both of these will be covered in later articles. But be warned that even single wires have a VF that, sometimes, needs to be taken into consideration.
And do NOT overlook the connectors! I have seen feed lines as short as 10’ made from good quality cable with a measured loss of 15 dB because the connectors were poor quality, installed wrong, or both. Usually both. We’re going to talk more about connectors later, but for now I’ll say that N-type connectors just ain’t all that. Yeah…I’m going to get flack for that, too.
I can’t overstate the importance of your feed line…it can make or break your Ham station. You can have the very best antenna possible and 1500 watts of RF output, but if you have 15 dB of loss in your feed line, your signal will be poor at best.
Loads
Your antenna is nothing more than an electrical load, just like a light bulb. In fact, there was a time when light bulbs were used as antennas, but that’s another story. Again, it’s usually easiest to match the impedance of the antenna, feed line, and signal source (your transmitter), but there are other, more advanced, ways of making this all work. For now, however, we’re going to stick to the simple, easy solutions that work.
One word of warning, however…just because WA1BCD has a Super-Whiz-Bang antenna and he can work South Africa on 70cm using 10 milliwatts, that doesn’t mean you can get identical results. There are huge numbers of variables that figure into how a given antenna system will perform at a given location. We’re talking about things like mounting height, quality of the ground system, the very dirt itself, physical things in the environment (like mountains and trees), feed line length, and many, many more. None of that “your mileage may vary” nonsense…your mileage WILL vary.
As an aside (and a teaser for a future article), there is one school of thought in physics that looks at an antenna as a matching network…it matches the impedance of the generator (transmitter) and feed line to the impedance of the air. Now, before your head explodes, don’t get all wrapped up around the axle on this right now.
One of the most fun things about antennas is that you can make your own. (True…you can make your own feed line, too, but it’s not fun and usually not worth the effort.) This can vary from very simple (like we’ll look at in a moment) to extremely complex. General performance will also vary as the simpler antennas tend to be small, compact, and of “Unity Gain”, that is, they have about the same gain as a ½ wave dipole, or 0 dBd. At the other end of the spectrum (again, no pun intended) are the very complex antennas that tend to be large, spread out, and have gains measured in double digits.
This simple antenna has the advantage of being “Unbalanced” and that means you can easily feed it directly with an unbalanced line like coax. No need for baluns, ununs, or other things with funny names. One thing that you will need is to create a “choke” to stop any of your signal from radiating from the coax shield. All that means is to take some kind of form about 4” in diameter (not too critical…I use oatmeal canisters but PVC pipe works well, too) and wrap the coax feed line around the form 5 or 6 times. Tape or glue the coax in place. This choke should be located within about 2 feet of the antenna feed point. You can do the same thing with a few ferrite beads on the coax near the feed point. In most cases, having a choke on your feed line will not hurt anything, but some designs (like an end-fed half wave) will not work right with a choke. Again, in most cases, it might not help, but it won’t hurt. In the case of a ¼ wave ground plane, the choke is a huge help.
¼ Wave Ground Plane (Figure 1)
Let’s look at one of the simplest things you can build, and that is the ¼ Wave Ground Plane. The parts list comes down to a single SO-239 panel mount UHF coax socket, four 4-40 machine screws about ½” long, four 4-40 nuts, twelve #4 flat washers, and some #10 or #12 AWG bare solid copper wire. The only tools you will need are a screwdriver for the machine screws, a wrench for the nuts, a pair of needle-nose pliers, an SWR meter for the desired band, and a soldering iron. This little antenna is effectively a ½ wave dipole where each element is about ¼ wave long mounted to give vertical operation (for now, don’t worry about horizontal and vertical polarization) that is perfect for working local repeaters on VHF and UHF. See Figure 1 for the details. You will cut some wire to the correct lengths, attach the four longer wires to the SO-239 using the #4 hardware, bend those radials down to about a 45 degree angle (keep them all as close to the same as you can), and then solder the remaining short wire into the center pin of the SO-239. A little dab of silicon rubber caulk on the soldered joint and connector insulation will keep rain out of the coax and connector.
The approximate dimensions for the wires are:
2m (146 MHz) A = 19-1/4” B = 21-1/2” (four of these)
1.25m (224 MHz) A = 12-1/2” B = 14” (four of these)
70cm (445 MHz) A = 6-1/4” B = 7” (four of these)
Personally, I always cut the wire a little long and then trim them back as I tune the antenna as described below.
Using an SWR meter check the vSWR. This antenna, when adjusted for the center of the band, should cover the entire 2m and 1.25m bands, and the upper 10 MHz of the 70cm band (the FM portion). If you wish to shift the resonant frequency, adjust the length of the upright element where longer will shift lower in frequency and shorter will be higher. Make VERY small adjustments to the length! The match should show close to 50 ohms, but you can adjust the antenna impedance by adjusting the angle of the radials…less than 45 degrees will be lower and greater than 45 degrees will be higher. Again, if you adjust the “droop” of the radials, try to keep all four the same.
As you can see, this is a very compact antenna…even the 2m variant is only about 3’ tall. You will have to work out a way to mount the antenna, but when I’ve built these in the past, I’ve done a little bit of everything including (but not limited to): Using a hose clamp around the PL-259 coax connector to strap the whole thing to a wooden dowel; Gluing the SO-239 to the end of a PVC pipe and running the coax down through the pipe; Using silicon sealant to glue it to something; Putting a small loop at the tip of the upright element and using a string to hang it all from a tree limb…I’m sure you can get creative! When mounting the antenna, keep in mind that ground plane antennas work best when at least two wavelengths above ground. For 2m, this would be about 7’ minimum, and, in general. VHF/UHF antennas should be mounted as high as possible. While I don’t think I would try it with the 2m version, the 1.25m and 70cm versions could be taken mobile with a little thought. I have even seen Hams with the 70cm version mounted to a hat and connected to their HT as they walk around Ham Fests and such.
As mentioned, this antenna is a unity gain system, but if you are using a handheld radio, this will have as much as 27 dB gain over your rubber duck antenna. Even compared to the high-end HT antennas, you’re still looking at gains in the 3 dB range. And remember that a 3 dB gain is doubling your signal.
There are many good sites online for calculating the dimensions of quarter wave ground plane antennas. My favorite is: https://m0ukd.com/calculators/quarter-wave-ground-plane-antenna-calculator/ where you can input your exact desired frequency. This is the site used for this article. If you do this, remember that you should enter the center of the desired frequency range. In theory, that will be the point of lowest vSWR and as you move higher or lower in frequency, the vSWR will rise.
Next time, we’ll take a look at the mysterious, and often misused, Standing Wave Ratio.
Take Care & 73
de KC6UFM
Charles