The perfect antenna is the holy grail of amateur radio. But antenna tuning is a game of inches, and since the optimum length of an antenna depends on the frequency it’s used on, the mere act of spinning the dial means that every antenna design is a compromise. Or perhaps not, if you build this infinitely adjustable capstan-winch dipole antenna.
Dipoles are generally built to resonate around the center frequency of one band, and with allocations ranging almost from “DC to daylight”, hams often end up with a forest of dipoles. [AD0MZ]’s adjustable dipole solves that problem, making the antenna usable from the 80-meter band down to 10 meters. To accomplish this feat it uses something familiar to any sailor: a capstan winch.
The feedpoint of the antenna contains a pair of 3D-printed drums, each wound with a loop of tinned 18-gauge antenna wire attached to some Dacron cord. These make up the adjustable-length elements of the antenna, which are strung through pulleys suspended in trees about 40 meters apart. Inside the feedpoint enclosure are brushes from an electric drill to connect the elements to a 1:1 balun and a stepper motor to run the winch. As the wire pays out of one spool, the Dacron cord is taken up by the other; the same thing happens on the other side of the antenna, resulting in a balanced configuration.
We think this is a really clever design that should make many a ham happy across the bands. We even see how this could be adapted to other antenna configurations, like the end-fed halfwave we recently featured in our “$50 Ham” series.
I think that this is probably how the fancy “Step IR” antennas work also.
My thoughts exactly!
EXACTLY! de Howie wa3mck
It’s a clever idea, but it sounds like the carbon brushes are at the center feedpoint of the dipole. That’s exactly the wrong place to have extra carbon resistors.
Could your two wires be permanently connected at the feedpoint? Make each wire a 1/4-wavelength at 80 meters. Connect the far ends of each wire to cords that come back to your drums. Then arrange a set of pulleys (block-and-tackle fashion) so winding up the cord shortens the antenna wires by doubling it back on itself. To get all the way down from 80 to 10 meters, the wire would have to be shortened to 8 strands in parallel (4 pulley pairs on each side).
I leave it as a exercise for the reader to figure out the support for the pulleys on the far end. :-)
I wonder if the drum was metallized and made the connection to the elements rather than the connection made via the car on brushes.
Metal rollers could also be used
This is certainly a neat project just needs to be made auto tune and your set
IMO, the carbon isn’t necessarily a high enough resistance to matter, think motor brushes, and secondly, metal on metal makes a heck of a lot of noise. So you would not be able to fiddle the length to peak the signal very dynamically, would have to adjust and wait.
The impedance in the center of a dipole is around 50 ohms. The resistance of a carbon brush from a small motor (like an electric drill) is a few ohms. Two of them in series could be a significant fraction of that 50 ohms. It could especially be troublesome if you were transmitting into the antenna.
There are huge carbon brushes in big motors, like golf carts and forklifts. They have far less resistance, though the contact area with the wire itself may become the limiting factor.
But as others have noted, antennas are all about compromise. Perfection is rarely achievable. Do whatever works. Anything is better than nothing, even if it’s a coat hanger stuck in the antenna jack! :-)
If they sized the drum right, they could slip on a tin can for a more durable and conductive surface.
That looks heavy – too heavy to put up on a pole at least a half-wavelength high, at least not without a mess of guy lines, certainly too heavy to self support using tension. A messenger rope (not wire) might work, but it must not expand/contract significantly with temperature, be UV resistant, and it needs to withstand an ice storm (even in Texas). That’s probably asking too much from a rope that is at least 134 feet (41 meters) long, a half-wavelength at 3.5 MHz.
Remember the part about every antenna being a compromise? I may be wrong but I really doubt that many people get their 80 meter antennas up 1/2 wavelength high. And yet they still manage to get out.
“And yet they still manage to get out.” Low dipole gets you Near Vertical Incidence Skywave (NVIS).
This is a pretty neat idea.
I’m not sure how well it would work but adding a home switch and encoder to the controller would allow the unit to run to preset positions.
When I get my Carbon Fiber Nylon 3D printer up and going, I may have a go at one.
Nice way to solve this tuning, I work on a solution to tune that way with an enfed antenna, this way to shorten and release and tune the antenna to 20, 30 , 40 meters… and make a perfect match.
This is what I love about my chosen hobby. Anytime someone come up with a solution to a common problem, fifty people come along and tell us how it won’t work.
Right? Like all those folks who told us a dipole in the attic “won’t work” or an end-fed along the top of a fence “won’t work”, or you need the absolute lowest loss coax/ladder line/etc money can buy or else it “won’t work”, etc etc.
Sometimes “good enough” really is good enough, and is a dang sight better than “not at all.”
Thanks, Jon.
With that I may actually manage to get something up for my new cross band TYT transmitter that’s stayed in the box for six months.
I interviewed Eric, AD0MZ in a live stream on youtube where we discussed this build in depth: https://www.youtube.com/watch?v=3XEsttr99uE
I can’t wait to watch this episode. Thanks