A Vertically Polarized Delta Loop for 40m
By Andrew Roos, ZS1AN
Having recently moved into a new house I found myself without a 40m antenna for use during the IARU HF contest. Although I am still awaiting permission to erect a tower, the garden of my new home has several moderately tall trees, so I decided to erect a temporary wire antenna to go with the paralleled dipoles for 20, 15 and 10m that I had already put up.
One obvious possibility was to put up another dipole for 40m. As a wise amateur once said, it takes a very good antenna to outperform a dipole. However all horizontally polarized antennas suffer from high radiation angles if the average height is much less than half a wavelength above ground. Although not a problem for local use, the higher angle of radiation compromises DX performance, a key requirement for any international contest. Half a wavelength on 40m is about 21m, so I measured my trees and found that the most suitable was only 12.5m (0.29 wavelengths) tall.
This led me to look for suitable vertically polarized antennas. Although they suffer more from ground losses than do horizontally polarized antennas, this deficiency is redeemed by their ability to emit a reasonable low-angle signal even when mounted close to ground.
The first option was a simple quarter-wave vertical. The radiator would be about 10.6m tall and could be suspended from the tree. However vertical antennas driven against ground are terribly inefficient unless a decent ground screen of 8 to 32 radials can be installed on or just below ground. This wasn’t possible in the time available. The need for a ground screen can be avoided by driving the antenna against a “ground plane” of quarter-wave radials raised 1/10 wavelength or more above ground, but it just wasn’t possible to install three or four 10m radials in my limited garden. Not to mention the danger to visitors and pets if the radials aren’t high enough to prevent inadvertent contact.
So I finally decided on one of the one-wavelength loop family of antennas. This family includes the quad and delta loops as well as the rectangle or “magnetic slot”. Although most commonly erected as horizontally polarized antennas, by changing the orientation or feed-point they can also be configured for vertical polarization. In this configuration they have the combined virtues of good low-angle radiation without excessive ground losses or the need for an extensive ground screen. The performance of these antennas has been extensively analysed by L. B. Cebik W4RNL in the excellent series of articles “Self-Contained Vertically Polarized Wire Antennas: A Family Album” that can be found on his website www.cebik.com.
The choice of which member of the family to use was based on practical considerations rather than performance. The available supports were the 12.5m tree, and my house about 25m away, with a height at the top of the pitched roof of about 8m. Fortunately the line from the tree to the house runs roughly West to East, so a loop antenna erected in this plane would radiate most strongly in the North/South axis, which tied in with my propagation predictions which showed that Europe would be the best target area for 40m during the contest. However because the house was significantly lower than the tree, I could not use an antenna design which required a long vertical axis on both sides of the loop. This immediately ruled out the magnetic slot and quad loop, as well as the related (although open-ended) half-square. However it was shaped ideally for the triangular delta loop, which could be suspended on its side between the house and tree so that the long vertical axis was on the tree side, and the feed point at the apex of the triangle on the house side.
Practical construction was simple. I tied one end of a length of nylon ski-rope to a decorative lantern on the east of the house (right-hand side in the diagram), and tied a couple of heavy fishing weights to the other end. I then threw the weights over the roof, having first ensured that the pets were safely shut inside and the YL out shopping. (A cute kitten brained by plummeting fishing weight is not a good way to endear our wonderful hobby to your life partner. Life partner brained by fishing weight is even worse.) I then threw the weighted end of the rope over a suitable branch in the tree and tied it to a stake embedded in the ground beneath the tree. The rope served as a support for the antenna itself, which was made of 1.5 mm diameter enameled copper wire. The bottom of the vertical side was tied to the tree using another short length of ski-rope to maintain the delta shape. (The ski rope is shown dotted in the diagram above, while the wire is shown as a solid line).
The antenna dimensions are approximately 17.8 m on each of the long sides, and 9.4 m on the vertical axis. These dimensions give a good match to 50 ohms with an SWR of less than 1.5:1 throughout the 40 m band. As with all antennas, you should initially cut it long and then progressively shorten it until resonance is achieved. The feed point is at the apex of the triangle, the far right hand side as depicted above. Although it is a balanced antenna and should ideally be fed with a balun, you can feed directly from 50-ohm coax at the risk of some pattern distortion. If you do feed it without a balun then it does not matter which leg of the loop is connected to the shield and which leg is connected to the inner conductor.
I constructed an EZ-NEC model that predicts a maximum gain of 1.6 dBi perpendicular to the plane of the loop at an elevation angle of 22°, and a gain of –3.0 dBi at an elevation angle of 6°, which is the angle suggested by Moxon as being representative of DX performance. While by no means spectacular, this should nevertheless provide better DX performance than a flat-top dipole at the same maximum height of 12.2 m above ground, which has a maximum gain of 5.8 dBi at 51° elevation (resulting in lots of local QRM) but is over 4 dB worse at 6° with a gain of –7.1° dBi. Both antennas were modeled over “Medium” ground using the Sommerfeld-Norton model.