A Convenience Store Based Antenna
Ham radio operators world-wide would love an antenna that is cheap, easily built out of locally available parts – and outperforms both standard dipole antennas and other commonly-built wires. Well, here it is….
After almost a year of modeling our ideas for a “Super Antenna” the design was locked. It was built and put on the air during the CQ Worldwide contest last week.
It was a pile-up buster. (To non hams: it kicked-ass!)
In a space of 1/2-hour, I logged Portugal, Aruba, Italy, and somewhere in ITU Zone 29 (roughly Azerbaijan. Plus, uncountable contacts state-side. And when these basic signal contacts were done, had a nice +30dB over S-9 on C.W. (amp on, NY area from Texas).
All on an antenna you can make for about $65-bucks and a few hours work.
The “Secret Sauce”
Every antenna builder/designer claims they have some “secret sauce.” I’m no different. EXCEPT that I have run my “secret sauce” through countless iterations modeling.
Setting up modeling software is important for proper evaluation. In EZ-NEC (pro), careful attention was paid to setting up both the ground conditions (Real- High Accuracy) as well as the reference antenna (2.15 dBi) which enables dBd (decibels relative to a free-space (isotropic) dipole.
>>>> The “secret sauce” after all these iterations? Antennas work best when there are multiple conductors on one side of a center feedpoint. Usually, a couple of dB!
At the center, all these additional wires are connected together. At the far end of the antenna, they’re 3 to 6-inches apart but NOT tied together. The effect disappears if you do that…
Walk-through of the Secret
With a simplest dipole (like for 20 meters), you have two pieces of wire, each ~16.7 feet long. These are hung in the modeling software (as a flat-top) at 50-feet. Signal source is attached: Center conductor to one side, braid or opposite side of ladder line to the other.
And it will work. Very well, though it depends on aim. You see, the traditional dipole antenna fires broadside (perpendicular) to the antenna. Since it has significant lobes when plotted, the antenna has up to 5.36 dBd in broadside directions, but will perform poorly 90-degrees to those lobes. The 5.36 db is “pulled in” from the end lobes.
The Dipole is not perfect, though. Significant energy is “wasted” going “straight up.” The effect (top hat pattern) is more or less pronounced depending on elevation above ground. Fine on 40 and 80 meters, where NIVIS (near vertical incident skywave) is desirable. But not on 20 where the best take-off angles are low to the horizon for long distance coverage.
The area in yellow, here, shows the “wasted” signal from a dipole that doesn’t help with long distance communications. Great stateside, but in Europe of Africa? Not so much.
Now, let’s take the exact same antenna elevation and tweak. We’re going to add a couple of wires on one side. All wires tied at the center, 3-inches apart at the ends (and not connected out there).
See what happens? Our performance for long distance work improves! A LOT! 1.48 dB. Effectively we pull from the (marginally useful top hat area and redirect it down toward the horizon.
What my crude drawing shows (Blue arrows) is that energy has been taken from the vertical lobe and has been redirected down to the more useful (for long distances) 20-degree take-off angle.
See? We pick up almost 1 1/2 dBd Adding two additional 16.7 foot pieces of wire.
Now let’s get to work optimizing! Spoiler alert! Using a single 49 foot run on the “hot” side and three 90-foot wires on the other this antenna begins to rock…
OCFD & Windom’s Work Better
Since the “secret sauce” is additional wires on the “cold side” the “regular dipole” becomes even more unbalanced. We can then exploit in the following design:
Which results in a marvelous signal on 20 meters! (And OK on 40, plus very good on 80…too!):
As should be evident, with the Off-Center-Fed Dipole (OCFD) (also called a Windom, by some, though a Windom uses a single -wire feed) we have “squished” the vertical lobe down and increased the low take-off energy…dramatically. In fact, here’s a performance comparison in table form:
This is based on the hot side wire being 49-feet long and the three cold side wires being 90-feet long. Different elevations will yield differences, but the AC7X Super Antenna wins every time.
I call this the 49-3X-90 design. 49 feet on one side, three times 90 feet on the other. It’s a convenient way to keep track of dozens of designs when optimizing. Just use a standard elevation and ground.
You Need Open Wire Feedline
Ladder line is fine, too: 300 or 450 ohms, your choice. 300 MIGHT be better, but barely measurable. Ladder line has much lower loss at high standing wave ratios than coax.
Before you run out and build one, realize this is the ONE fly in the ointment: while it will work dandy on 3.5 and 14 MHz band with a 2:1 balun, you will need an antenna tuner and open wire feedline to nail 40. This is because on 40 meters, the antenna impedance is around 273 ohms.
On 20 meters, a 2:1 balun will give under 2-to-1 SWR across the whole band. But not get better than 3 1/2 to on 40 without “help” which is why the ladder line tuner.
On the other hand, as a high-performance (and good gain) antenna on the 20-meter band, the antenna is hard to beat. Better than my beam (*which has a broken trap) by far! From the bottom of 20 (1.85 : 1) though the top (1.54 : 1), the SWR is flat at 14.2 MHz. You can’t beat them numbers with a stick!
Convenience Store Insulators
The first version I put up employed a commercial ladder line center insulator. Broke in the first good wind. Use of 20 percent infill in innards of the plastic molding was the cause of failure identified.
This means you will want to use something more solid. A super-tough plastic like Delrin is great. But, have you priced that stuff? Wow!
A trip to the local convenience store paid off, though. A super-tough piece of antenna insulator, one-half inch thick, is widely available. Although people buy them mistaking them for 1/2″ to 3/4″ thick “cutting boards” not as antenna insulator raw stock. Go figure. World’s full of heathens, though, ain’t it?
Off to the shop, where we rip off the rounded edges…
Which led to Learning #2: Table saws leave ugly melted plastic sticking to the edges:
You can peel most of this off by hand easily enough. Some touch-up on the belt sander brings it back to useable.
For following cuts, I was bright-enough to use the band saw. You will want to lay out your “cutting board insulator” something like this:
Yeah – I know. Not a “standard looking” insulator. But, here’s another “crackpot theory” for you:
On most antenna insulators, people twist wires “back on themselves.” I know “the books” say this doesn’t matter, but instead of “back on themselves” I chose to set up a “belt” approach for all wires.
The wires go in the outer hole, around the back, and out the next one. This way, there is no “conductor reversal.”
Same thing with the ladder line: Goes through the three belt-like openings which were cut on the milling machine.
Just before assembly, the insulators looked like this:
At the center insulator, everything ties together at the two bolts. They go immediately to holes about 5/8ths inch apart. On the “special side” of the antenna, there are spacers every 20-feet, or so to keep things from twisting up too much. It wants to without them.
To keep the spreaders in place, I put a large drop of 5-minute epoxy at each spreader-wire joint on the top wire ONLY. I’ve tried small zip-ties, but they slide. These will over time – if you show up with a D-6 Cat to pull ’em…
The wire used is el cheapo electrical supply house (or eBay overstock 500′ rolls are cheap) #14 stranded THHN. Sautéed lightly in rosin flux and then tossed into a telegraph splice if needed. On the ends, sautéed wires are crimped into ring terminals that land on stainless bolts. After being properly dressed with solder flowed into each. (Can you guess who was hungry when writing?)
So there you have it: The AC7X Secret Sauce Antenna – the 49-3X-90 – and how to build one on the cheap.
When I get some time in TinkerCAD, I will draw up something more elegant – designed for printing in ABS – and will put it up as an .STL file both here and over on our https://ultra-make.com 3D printing site.
That design will likely have additional “belt loops” and I’ll try to fit it all in the 220 mm X 220 mm footprint of the Ender 3 type printers. Not everyone has a CR-10 MAX yet. But you do believe in Printer Claus, right? Make sure Printer Claus lands on your tower before the next lock down and brings? Filament and antenna wire, Rudolph?
Oh…and Dummy loads for people in W-6 and W-2 land.
Write when you get rich (or make DXCC),