Speaker Wire Delta Loop

Here’s an example of what can happen when you have a hunk of cheap wire and a little too much time on your hands.

Years back, I did a write-up on a simple, random wire antenna made from a 50-foot roll of speaker wire from a local dollar store. I nick-named it the Dollar Store Special. I had a similar roll of wire in my junk box, so I set out to see if I could build another useful portable antenna from it.

This time out, I wanted to build something more elaborate than a random wire. After some sketching with a pencil and paper, I came up with this simple portable delta loop.

There are certainly better ways to construct a delta loop. However, I just wanted to see if I could build a functional antenna using only cheap speaker wire. So, with that in mind, here’s how I did it.

The Design

The antenna I built was inspired by a portable delta loop designed by Doug DeMaw, W1FB. [1] Doug’s multiband delta loop was designed for the 40M band and used a 300-ohm balanced feeder. 

According to Doug’s book, this type of antenna should work well on the fundamental frequency and higher. For the next band below the fundamental, he suggests connecting the feeder wires together and using it like a random wire. I figured I’d just try loading it up as is to see what happens.

Given that I constrained myself to a 50-foot roll of speak wire, I scaled my antenna for the 20M band. Using the formula, 1005 divided by the frequency in megahertz, I calculated a total length of 71 feet (21.6 meters) for the center of the 20M band. That would leave some of the two-conductor wire for an improvised balanced feeder.

Feeding the delta loop in a corner (with the apex of the loop pointing up), gives the antenna vertical polarity with a low take-off angle.[2] As with most antennas, higher is better. However, this antenna is still quite useful at practical heights in the field.

Since a tuner will always be necessary, I expended no effort trying to optimize the design.

Construction

If you’re a visual person like me, refer to the diagram to help make sense of the directions below.

1. Measure off 35.5 feet from one end of the speaker wire. Place a small zip-tie around the wire at this point.
2. Separate the 35.5-foot end of the speaker wire into two separate wires.
3. Strip and solder the loose ends of the 35.5-foot wires together. Put some electrical tape or shrink tubing over the splice.
4. Make 3 small loops in the wire, as shown in the diagram. You can see an example in the accompanying photo. These are going to be the attachment points. I used some Goop® adhesive on the zip-ties to help hold things in place.
5. Finally, install some spade terminals on the ends of the shorter conductors. These will be used to attach the antenna to your tuner or balun.

Deployment

For my initial tests, I used a 28-foot Jackite pole to support the antenna. I only partially-extended the pole, such that the bottom of the antenna was about 4 to 5 feet off the ground. I used some nylon twine and a couple of tent stakes to tie off the two bottom corners.

The setup was somewhat more complicated than most portable antennas I use. It took me about 20 minutes to get it set up, but I suppose that wasn’t too bad for my first time. 

I used a couple of large tent stakes to keep the feedline off the ground. I connected the antenna to my KX3 using a 4:1 balun and a 1-foot piece of coax. 

Results

I first did a quick check to see what bands the KX3’s internal antenna tuner would handle. I found that I could load it up on every band from 60M through 6M, although I couldn’t get the SWR below 2:1 in the low end of 40M. That’s not surprising for a 20M loop, I suppose. I did have a usuable match between 7.030 and 7.060, where I normally operate.

I was only about 50 yards away from some powerlines, but the loop seemed quiet on receive. 

On 20M, a French station answered my third CQ. I also made contacts with Missouri and wrapped up with yet another French station. 

From the signal report the last station gave me, this antenna appears to do reasonably well with DX on 20M running QRP. It was a chilly and windy day, so I didn’t stay out there to try for contacts on other bands. 

Wrap-Up

Although my initial outing with this antenna was promising, I need to spend some more time using it on bands other than 20M. In any event, it was a fun—and cheap—antenna project.

73, Craig WB3GCK

References:

[1] DeMaw, D. (1991). Technical Bits & Pieces. In W1FB’s QRP Notebook (2nd Edition, pp. 157–161). Newington, CT: QST.
[2] DeMaw, D., & Aurick, L. (1984, October). The Full-Wave Delta Loop at Low Height. QST, 24–26.