An Outside the Box version of the Delta Loop antenna

I have written about delta loop antennas before – notably in April 2023 in a post entitled “Delta Loopy Ideas“. A few weeks ago I received an email from Alan WA3EKL who suggested I should take another look at delta loops and offered a lot of helpful advice, tips and suggestions for making this type of antenna a success. Alan’s suggestions have been a tremendous help in designing this latest iteration of my rapid deployment, field expedient, portable delta loop antenna for the 20m band.

Before we go on to describe what is definitely an “outside the box” variant of the delta loop I should comment that I remain welded to the idea that delta loops can be tricky to set up. There are so many variables to consider:

• orientation (apex up, down or sideways)
• feedpoint (at the apex, at a corner, a quarter wave from the apex or somewhere in between) giving horizontal or vertical polarization
• loop length (1005/wavelength or some other length, bare or insulated wire)
• equilateral triangle (or not)
• height of wire above ground
• Matching device (quarter-wavelength 75 ohm coax / impedance transformer / L-network / tuner)

On the positive side you can simplify all the variables and just throw up an “inside the box” version. You will make contacts – but will the standard, don’t-bend-the-rules version of a delta loop be as efficient as possible?

My personal quest has been to find an antenna that is efficient enough to let me carry on working QRP during the Sun’s current spate of hissy fits. With that in mind I wanted to experiment with the variables to see if every last picowatt of RF energy could be transferred from my humble, so-small-its-hard-to-see-without-a-lens QRP-Labs QMX transceiver to the ionosphere via whatever variant of the delta loop became necessary. I have a muscle radio that could get the job done, but there is an engineering and physics challenge built into this quest. Part of the challenge of QRP is to use brains over brawn.

So let’s examine those design variables and see what emerges from the mist. First up:

1. Orientation (apex up, down or sideways)

There are arguments to be made for and against each way of orienting the loop – and let’s emphasize that we are discussing only vertical orientations here. This is not meant to be a treatise on the basic theory of delta loops, so let me just state that only one orientation fills my needs. The use case we will discuss here is a rapid deployment, field expedient, portable antenna. The antenna must be lightweight, pack into a very small space and will remain erected for perhaps only an hour – just long enough to complete a POTA activation.

The orientation that fills this need is an apex up triangle supported by a telescoping fiberglass pole (a 7m Spiderbeam) at the center. Each end of the bottom section of the loop is supported by a modified trekking pole stuck in the ground.

The Spiderbeam pole could be omitted if a tree limb is available for the center support. A fence or bush could replace the trekking poles. I prefer to be self-sufficient out in the Big Blue Sky Shack and not depend on whatever might be available at the end of the trail. Trekking poles can also be used for their intended purpose en route to the operating site.

2. Feedpoint

2:1 impedance transformer at feedpoint

A delta loop can be fed at the apex, at a corner, a quarter wave from the apex or somewhere in between. I chose to feed my loop at a corner for convenience. The choice of feedpoint affects whether the radiated signal is vertically or horizontally polarized. Frankly, I don’t care; the radiated signal is going to be fired up to the periphery of the “Final Frontier” where it will be kicked around by the ionosphere’s D-layer, then refracted back down to Earth with who-knows-what polarization. We might be more concerned at, say, a Field Day site where it might be desirable to chose vertical or horizontal polarization to avoid interference in the near field with other stations.

3. Loop length

Far end of antenna supported on a trekking pole

Those who don’t wish to rock the boat of antenna orthodoxy will just cut their wire using the formula 1005/wavelength. Should the wire be bare or insulated? Does it matter? Some sources say absolutely not. Other sources suggest the effect of insulation is finite, but insignificant.

I decided to consult the stone tablets to get an official view of the effect of wire insulation. First I consulted the EZNEC manual in which EZNEC creator Roy Lewallen W7EL suggests the difference between bare and insulated wire is very small – perhaps 2-3%. EZNEC allows users to specify wire insulation in its calculations. If we consider that a 20m delta loop has a nominal wire length of 71.5ft, even a 2% impact could change the wire length by almost 18 inches. Significant? Perhaps not, but when I explain how I derived the wire length of my Outside the Box delta loop you may understand how my thinking was swayed on this issue.

Bottom wire section center secured to pole with a Canadian Jam Knot

One source may not be enough to establish a rule, so I also consulted the excellent website portable-antennas.com from Rob DM1CM. Modeling a delta loop on that site was a simple exercise of selecting various drop-down selection boxes – fast and easy. Portable Antennas.com also allows wire insulation to be calculated into the loop design. After plugging in the numbers for my loop the model gave a correction factor of 0.9754 when insulated wire is used. Based on a 71.5ft loop length that would suggest a shortening of 1.76 feet even using just a 2% shortening factor.

How did I determine my loop length? I made the decision to choose the loop length by trimming the wire until I obtained resonance in the CW QRP portion of the 20m band. I set my RigExpert AA55 Zoom antenna analyzer to plot an R,X (Resistance and Reactance) chart. The wire was then trimmed until the RigExpert showed an X value of zero. The wire length? I laid the wire out on the lawn I had neatly cut the day before and measured the length – it turned out to be just 68 feet. If the shortening factor of the wire insulation is taken into account the corrected length would be between 69.36 and 70.1 feet. Is that significant? Perhaps; if the resonant frequency were changed by 2% it might no longer lie in the desired part of the band.

4. Equilateral triangle? Height above ground

Ideally a loop antenna should enclose as much area as possible (source: well, I read it somewhere). My design was influenced by two factors – the height of the Spiderbeam pole (7m, 23ft) and the recommended height of the bottom section of wire above ground (source: Alan WA3EKL recommended between 4 and 6 feet; I chose 4 feet). So the Outside the Box variant of a delta loop is not an equilateral triangle. The bottom section of the wire (the hypotenuse) is longer than the other two sides. Less efficient than an equilateral triangle? Piff!

5. The matching device

Alan WA3EKL suggested a quarter-wavelength section of 75 ohm coax to match the impedance of the loop to 50 ohms, but cautioned against the use of foam dielectric coax whose velocity factor can change over time which affects its electrical length. The only 75 ohm coax I had available is RG-6 which has foam dielectric so I ruled that out.

I recently watched an interesting YouTube video by Michael KB9VBR, who was discussing his way of matching a delta loop antenna. Michael used an impedance transformer with an impedance ratio of 2.5:1 (8:5 turns ratio). That seemed like a convenient and field expedient way of matching the delta loop impedance of 100-120 ohms to 50 ohms so I built one. Out in the antenna test range (my backyard) it was discovered that a 2:1 ratio worked best (turns ratio 7:5).

The final result

My Outside the Box delta loop antenna is resonant at 14168 KHz (slightly higher than planned, but not a problem). The minimum SWR is 1.03:1 at 14175 KHz. At the lower band edge at 14000 KHz, the SWR is 1.15:1 and at the upper band edge of 14350 KHz, the SWR is 1.11:1. And, yes, I have made contacts with it. I bent the rules but the end result is worthy of making the trip to the field this summer.

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