Can a non resonant antenna be efficient? It sure can. I show you how to build a non resonant random wire antenna with a 9:1 transformer.
You may be quite familiar with the end fed half wave antenna. That’s a very popular and common antenna that consists of resonant half wave of wire for your lowest desired frequency and a transformer at the feed point. Moving the feed point all the way to the end of the dipole, creates a very high impedance of about 4000 to 5000 ohms, which necessitates using a 49:1 transformer to bring us back to a 50 ohm match. This method works well and gives you multiple band coverage on the fundamental frequency and even and odd harmonics, usually without a tuner.
But there is another method of creating an end fed antenna, and it’s one of the oldest styles in the book. That’s the random wire end fed antenna.
Historically, the random wire antenna, also called the end fed Zepp due to its use on Zepplin dirigibles, dates back to the earliest days of amateur radio. A piece of wire is fed at its end by open feed parallel wire. If the wire is non resonant on the bands you wish to transmit on, the impedance at the end of the antenna will be quite low, about 450 to 600 ohms. This makes it easy to feed with 600 ohm ladder line which a Tube based transmitter could match this load and you would be on the air.
With modern transceivers, you can plug your random or non resonant wire into a wide range antenna tuner with the same effect. This makes the random wire one of the easiest and cheapest antennas to deploy. We demonstrated that capability in my recent Homebrew Antenna Challenge video, you’ll find a link to that video in the description down below.
Another option, and maybe a more convenient one, is to feed the antenna with coax and use the antenna by adding a 9:1 transformer at the feed point.
What makes this antenna unique? The key is in the wire length. If the wire is not resonant on any of the amateur bands, it will actually exhibit a lower impedance in the amateur bands and a higher impedance outside the amateur bands. Typically this impedance is about 450 to 600 ohms inside the amateur band, so they are relatively easy to match with a tuner or a simple to construct transformer.
How do we know how long of a wire to use? The simple answer is that you don’t want to use a wire length that is a ¼ or ½ wave multiple on any of the HF bands you wish to communicate on. Fortunately, smarter people created simple computer programs to do the math and filter out the good and bad wire lengths. You can look at a chart and pick a wire length appropriate to the number of bands you wish to operate on. For good 10 through 80 meter coverage, you’ll find that a non resonant wire of about 74 feet is ideal. This sure beats the 133 feet necessary for an 80 meter end fed half wave. Less wire makes for an easier to install antenna in tighter, size compromised locations. We can also hang up less wire for a portable operation as a 40 meter random wire could be as short as 33 feet long. We’ll talk about more advantages and disadvantages in a bit.
So random wire antennas aren’t very random at all, they are just carefully prescribed lengths of wire. But you have so many non resonant lengths to choose from that their actual size seems random.
What do we need to put a random wire antenna on the air? I’m going to go through the process of building a 9:1 transformer, or UnUn, so we can feed the antenna with coax. This is the transformer I built for the home brew antenna challenge, I going to do the same thing, but place it into this weather resistant box.
Roughly stated, to build the transformer, you will need a ferrite core and wire. One of the most common methods to build a 9:1 transformer is to wind 9 turns of wire in a trifilar winding around a torroid core. This means that all three wires are simultaneously wound around the core.
For the core, there are many different types of ferrites available. I’m using a Fair-Rite brand FT-140 type 43 mix ferrite. When shopping for ferrites, buy a reputable brand so you know what the composition is, as some resellers may not properly label or identify their products. Links to the Fair-Rite torroid are down below. Type 43 mix is good for wide band HF operation and the medium sized core is adequate for 100 watt sideband and CW operation and probably about 50 watt digital. For wire I’m using 18 gauge enameled magnet wire, again adequate for 100 watts operation. If I want to operate at higher power levels, I can use a larger core, stack two cores and use a heavier gauge wire. The two things that really affect transformers are RF saturation and overheating. More core material will reduce both of those effects.
Let’s wrap the core. Toroids.info has a calculator that tells you how much wire to use for the ferrite. For a 9:1, I’ll need three paired wires wrapped 9 times around the core. The calculator tells me about 19 inches of wire for an FT-140 core. I want a bit more tail at the end, so I increased that length about 24 inches of wire.
Since I’m using enameled wire, it is recommended to wrap insulation around the core. Electrical tape or other insulating material works well. This will slow heat dissipation and slightly increase your wire length, so keep that in mind.
Hold all three wires and feed them through the core, taking care to keep them from crossing over. While crossovers won’t really affect the performance, it does make for a neater wrap. Do this nine times. Each time the wire goes through the center of the core counts as a turn.
After wrapping the core, use sandpaper to rub the enamel off the ends of the wires. The enamel acts as insulation so we will need to remove it to hook everything up. After that task, I will use a meter to check the continuity of each wire and mark them with tape. Each wire end is marked A1 B1 C1 and A2 B2 and C2. This aids in the hook up.
To hook up the antenna, I’ve prepared this box. It’s an outdoor rated conduit box that’s available at your local Home Depot. I drilled holes in the box, ¼ inch for a hook on the top and antenna and counterpoise connections on the sides and an ⅝ inch hold for the SO-239 on the bottom. A step drill bit is really helpful for that. This box is roomy enough for a stacked transformer or even the large FT-240 core, so there is plenty of space for the smaller FT 140 core.
Use the chart to hook up the antenna. A1 goes to the antenna connection. B1 and A2 are connected together. C1 and B2 are both connected to the center pin of the SO-239 and C2 goes to the ground or counterpoise along with the braid of the coax.
Once everything is connected, how do we know it works? I like to take a couple of resistors, I have two 270 ohm resistors wired in series for a 540 ohm load, and I place it on the antenna and counterpoise connectors on the box. If the transformer is wired correctly, with an antenna analyzer you will see a constant low SWR across the entire HF spectrum.
My transformer appears to be working, so let’s go outside and get it on the air.
So what are the advantages and disadvantages of a non resonant antenna. First off is space. You can put up a 10 through 40 meter antenna with 33 feet of wire and an 80 meter antenna with 74 feet. If you want to get on 160 meters, just increase your wire length to 135 feet. So it’s a very good, space saving antenna. That feature alone makes it desirable for those with limited lot sizes, those wanting a stealth antenna, or a space saving portable antenna.
Advantages and disadvantages
You can mount these antennas in a variety of configurations, sloper, flat top, inverted L or inverted Vee. Power handling is only limited by your transformer size, so they are good from QRP to QRO.
But there are also down sides. First off is the efficiency. Any antenna that requires a transformer will have efficiency losses. Transformers generate heat, and heat is lost energy. These antennas require a counterpoise, so that consideration will need to be made in your deployment.
Non resonant antennas can also present RF in the shack. That’s not much of an issue if you are coax feeding the antenna, but if you are using ladder line, direct feed from a tuner, or have a poor ground or counterpoise, RF can make its way into your system. Just be aware of that.
So, is a nonresonant or random wire antenna a good choice for you? It depends. There are many factors that dictate antenna selection, like band choices, available space, HOA or other restrictions, and portability. Antennas are just a tool and there is no one size fits all solution. But the random wire, or non resonant antenna shouldn’t be dismissed as a poor or compromise antenna. It has a long history and has stood the test of time as an effective radiator. Building the transformer isn’t difficult, so give it try. You may find it workable for your needs.
FT-140 43 mix Ferrite Toroid: https://www.mouser.com/ProductDetail/Fair-Rite/5943002701
Toroid Core FT140-43 Ferrite: https://amzn.to/3WKPApC
18ga Enameled Magnet Wire: https://amzn.to/3WFhSBX
4 in. x 4 in. x 2 in. PVC Junction Box – Gray: https://www.homedepot.com/p/Carlon-4-in-x-4-in-x-2-in-PVC-Junction-Box-Gray-E989NNJ-CAR/100404097
Homebrew Antenna Challenge:
Random Wire end fed antenna resources:
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00:33 End Fed Antennas
01:22 Random wire antennas: long wire or end fed Zepp
03:11 Wire Lengths are important
04:09 Ferrite cores and wire
06:55 Building the 9:1 transformer
09:28 Installing the antenna
11:05 Testing and making contacts
15:12 Advantages and Disadvantages of non resonant antennas