Several members of the Nashua Area Radio Club recently got together to help one of our members to put up a 40m Delta Loop antenna. Delta loop antennas are some of the best-performing wire antennas and have the advantage that they can be easily supported via a single elevated mounting point such as a mast or a tall tree.
The location we chose for the 40m Delta Loop already had a radial field under it, and we decided to take advantage of this to try to improve ground quality under the antenna. (A radial field is not required for this antenna – the author has built several Delta Loop antennas for 40m without radial fields, and they all performed very well). We built a detailed EZNEC model of the planned antenna and radial field under it to evaluate the best approach to the design. The antenna was modeled and built with its apex around 50 ft, and designs resulting in Horizontal Polarization (feed point in the center of the bottom leg) and Vertical Polarization (feed point 1/4 wavelength down a vertical leg) were considered.
The graphic above shows the EZNEC modeling result for the Vertically Polarized 40m Delta Loop. This design tends to concentrate the radiated energy at the lower takeoff angles, which optimize the antenna for long DX contacts.
This graphic shows the modeled pattern when the antenna is fed to create Horizontal Polarization. At first look, one might conclude that this version of the antenna would only be useful for short-range communications. The point to consider here is that the Horizontally Polarized version of the antenna has a much higher overall gain.
The table above helps to better understand the real difference in performance between the two versions of the 40m Delta Loop. Columns 2 – 4 compare the gain of the two versions of the antenna at various elevation angles. We can see that the Horizontally Polarized version of our 40m Delta Loop has a higher gain down to elevation angles of about 20 degrees. At lower elevation angles, the Vertically Polarized version has an advantage. Also, note that both antennas begin to exhibit less than -1.0 dBi of “gain” at angles below 10 degrees of elevation. The net of this is that the Vertically Polarized antenna does have an advantage for DX signals which arrive at low angles in the 20 to 10-degree range. Such low angles would be typical for very long DX contacts or marginal propagation conditions, which might occur toward the bottom of the sunspot cycle. For many typical DX contacts (ex., DX contacts with Europe from here in New England) with arrival angles in the 20 to 30-degree range, the Horizontally Polarized version of the antenna will probably perform better. Our 40m Delta Loop antenna is installed in a residential neighborhood with many potential noise sources close by, most of which will tend to be vertically polarized. The Horizontally Polarized version of the antenna will be less sensitive to these local noise sources, which gives it a further advantage in this situation.
It’s also interesting to note the effect of the radial field under the antenna in the table above. The benefit of radials is pretty limited in the Vertically Polarized configuration averaging less than 0.5 dB. The gain from the radials is more significant in the Horizontal configuration averaging close to 1.5 dB. This is enough to expose an additional “layer” of weaker stations.
There is a common misconception that a Delta Loop Antenna is directional toward the open side of the loop. This is not the case at typical heights above ground. The plot above shows the pattern of the Horizontally Polarized version of the loop to be omnidirectional.
The graphic above shows the equivalent pattern for the Vertically Polarized version. Note that this antenna is slightly directional off the ends of the antenna, not towards the open side of the loop. This is a result of the fact that the two vertical legs of the Vertically Polarized Delta Loop antenna behave somewhat like two independent vertical antennas in a phased array.
The net of this analysis was that we decided to build the Horizontally Polarized version of the 40m Delta Loop.
The first step in constructing the antenna system was to expand the existing radial field under the antenna. We next put up a 50 ft guyed mast to support the loop’s apex. The result is shown above.
The antenna’s bottom wire element is about 10 ft above the ground, and the bottom corners are anchored to the ground via Dacron guy ropes. After trimming the antenna to be resonant in the center of the 40m band, we found the final impedance to be around 80 ohms. We used a custom 1.5:1 Balun from Balun Designs to create a 50-ohm match at the Delta Loop’s feed point. The picture above shows the Balun mounted on the mast about 10 feet from the ground. The antenna’s 2:1 SWR bandwidth with the Balun covers the entire 40m band. The antenna is fed with a relatively short run of LMR-400UF coax inside an underground conduit.
The project’s final step was to hook up the completed 40m Delta Loop system to a FlexRadio 6700 SDR. We made several QSOs with the new antenna to verify that its performance across the band was as expected. The setup has received good signal reports with the new antenna, confirming that it is performing as expected.
I have built several 40m Delta Loops at my home QTH as part of our club’s 2015 Field Day Operation this past year. All have been good performers. These antennas will usually also work on the 15m band without a tuner, and most will also work on the 20m with most Transceivers built-in tuners.
– Fred, AB1OC