Delta Loopy Ideas

It’s Spring in Southern Ontario. Maybe. The weather in these parts is highly unpredictable. Temperatures have been above freezing during the daytime and the snow is almost gone. But maybe it’ll be back. Who Knows? Whatever happens I have been taking advantage of the good weather for some antenna planning.

Antenna planning to me means getting out into the backyard with radio, wire, wire cutters and strippers, a pole (strippers … a pole? Really?) and trying out new ideas to see what works and what gets relegated to the junk box. My quest has been to find a new wire antenna for field portable work. The ideal antenna will be multi-band, resonant, lightweight and most definitely home-made – I don’t buy wire antennas.

Multi-band AND Resonant?

My tried and tested old faithful End-Fed Half Wave for 80m gives me 80m, 40m, 20m and 10m with less than 2:1 SWR. It has seen the top of a lot of trees over the last few years. But it’s time for a change methinks. Candidate #1 in the backyard interview process for the job of 2023 high wire act is the delta loop.

It Ain’t Multi-Band John!

Yes, you’re right. I suppose a delta loop might be “multi-bandable” if fed with open wire line. I haven’t tried it and probably never will. Frankly, delta loops already have a lot of variables that change their performance quite dramatically – as I discovered chez moi. Why add more complications?

Will the Circle Be Unbroken?

Now that solar cycle 25 has given some good propagation, 20m has been my goto band out in the field. A 20m delta loop is relatively compact so that is what I built. I used the formula 1005/Frequency to select the wire length of 71.5 feet and erected it in the shape of an equilateral triangle with the sharp end at the top supported by a fiberglass pole. “Equilateral” means all three sides are of equal length. This orientation provides a fairly low apex (for stealth in campgrounds). It is also the closest approximation of a circle that can be achieved with a triangle. Some sources suggest a circle is the ideal shape for a loop antenna. I don’t remember why.

Where to Feed the Beast?

A delta loop has multiple possible feedpoints and the choice has to be made very carefully indeed. I started out by placing the feedpoint at one corner. Feeding the loop at one corner provides the lowest radiation angle. The SWR was 1.8:1 and no matter how much I tinkered with the wire length that stubborn 1.8 number wouldn’t budge. Some sources suggest feeding the loop a quarter of the way up one of the sides. Tried it. The SWR finally budged – but the wrong way.

Then I remembered an antenna that a fellow club member had bought at a cost of hundreds of dollars. It was the Chameleon CHA TDL – Tactical Delta Loop. He erected it last Field Day as an upside-down triangle with the feedpoint at the sharp end of the triangle pointing down. I remember looking at the specs for the CHA-TDL and being confused. It is a beautiful piece of engineering but the radiation pattern didn’t seem right. Nonetheless, I thought, the engineers at Chameleon Antennas must have done their homework.

Up, Up and Away!

So, I moved the feedpoint of my home-made delta loop (cost was flat zero, by the way) to the center of the horizontal wire at the bottom of the antenna. Bingo! The SWR dropped a few points. Keeping in mind that dummy loads have a perfect SWR, I called a couple of POTA stations. An activator in Illinois gave me a 229 report for my 5 watt signal. Another station in North Carolina gave me a 559 report. Propagation wasn’t bad that day but these were not the kind of reports I was used to getting even with a simple quarter-wave vertical.

I checked out the antenna on my laptop using EZNEC and the answer became clear. Feeding a delta loop at the pointy-end/apex, or in the center of the horizontal wire perpendicularly below it results in very high angle radiation. Some of the signal squeezes out 3dB below at a lower angle, but 3dB below 5 watts is only two and half watts. Glad I didn’t spend hundreds of my dollars to find that out!

By the way, I am not trying to impugn the engineering of Chameleon’s CHA TDL. They have designed a very good antenna that meets the needs of many amateur radio operators, but I wanted something a little more focused on my specific operating needs.

I spent quite a long time messing with EZNEC modeling of the delta loop. It’s a helpful tool but, perhaps due to my inexperience using it, often doesn’t give quite the same results as getting out into the field and getting wire in the air.

We’re Gonna Get High

The next variable of this very variable antenna is its geometry. Maybe an equilateral triangle wasn’t the best orientation after all. Rapid deployment field antennas are often used in high visibility public areas, so it makes sense to occupy as little real estate as possible. Ground footprint is important; vertical footprint a little less so. Very tall poles may attract unwanted public curiosity and suspicion but kids and dogs aren’t going to trip over wires that are up high in the air.

So I raised my pole up as far as it would go (about 29 feet) to create a very elongated antenna shape. That narrowed the horizontal part of the wire by several feet. Another revelation; the antenna gods liked this style and blessed me with an even lower SWR. Even better, the SWR remained stable at around 1.3:1 no matter what small adjustments I made to the wire length. This here delta loop liked to breathe the fresh air way up there at the top of the pole. Had I found a strong candidate for my new field portable simple wire antenna?

Oh No John, No John, No!

Once again, I performed the “dummy load test” and made a QSO to check it out. I received a 339 report from Colorado. My enthusiasm was bruised. I took another look at my creation and decided any possible advantage over a simple quarter-wave vertical just wasn’t worth the added complexity of erecting a delta loop.

So, the search continues. The VP2E I previously blogged about, and which has proved itself on successful POTA activations, has earned and keeps its place in my field antenna bag. The VP2E is a monoband antenna so the search continues for a multiband candidate to join it. I have an idea what that might be and have already been successfully experimenting with it. Follow this blog or check back soon for the big reveal.