Building a 40m Delta Loop Antenna


40m Delta Loop Antenna

40m Delta Loop Antenna

Several members of the Nashua Area Radio Club recently got together to help one of our members, Ralph N1UH, 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 tall tree.

40m Delta Loop EZNEC Model

40m Delta Loop EZNEC Model

The location that Ralph choose for his 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. He 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 build 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.

40m Vertically Polarized Delta Loop Pattern

40m Vertically Polarized Delta Loop Pattern

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 optimizes the antenna for long DX contacts.

40m Horizontally Polarized Delta Loop Pattern

40m Horizontally Polarized Delta Loop Pattern

This graphic shows the modeled pattern when the antenna is fed to create Horizontal Polarization. On 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 much higher overall gain.

40m Delta Loop Gain Comparison

40m Delta Loop Gain Comparisons

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 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 – 10 degree range. Such low angles would be typical for very long DX contacts or in conditions of marginal propagation 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) which would have arrival angles in the 20 – 30 degree range, the Horizontally Polarized version of the antenna will probably perform better. Ralph lives 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 the 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 that 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 addition “layer” of weaker stations.

40m Horizontally Polarized Delta Loop Azimuth Pattern

40m Horizontally Polarized Delta Loop Azimuth Pattern

There is a common misconception that a Delta Loop Antenna is directional towards 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.

40m Vertically Polarized Delta Loop Azimuth Pattern

40m Vertically Polarized Delta Loop Azimuth Pattern

The graphic above shows the equivalent pattern for the Vertically Polarized version. Note that this antenna is actually slightly directional off the ends of the antenna, not towards to 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 all of this analysis was that Ralph decided to build the Horizontally Polarized version of the 40m Delta Loop.

Finished Radial Field Under The Antenna

Finished Radial Field Under The Antenna

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 apex of the loop. The result is shown above.

Feed Point and Balun on Mast

Feed Point and Balun on Mast

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 ft up from the ground. The antenna’s 2:1 SWR bandwidth with the Balun covers the entire 40m band. The antenna is fed with a relative short run of LMR-400UF coax inside underground conduit.

40m Delta Loop in use with FlexRadio-6700 SDR

40m Delta Loop in use with FlexRadio 6700 SDR

The final step of the project was to hook up the completed 40m Delta Loop system to Ralph’s FlexRadio 6700 SDR. We made several QSOs with the new antenna to verify that its performance across the band was as expected. Ralph has been receiving good signal reports with his new antenna further confirming that it is performing as expected. I have built several 40m Delta Loops at my home QTH at as part of our club’s 2015 Field Day Operation this past year. All have been good performers.

– Fred (AB1OC)

4 thoughts on “Building a 40m Delta Loop Antenna

  1. Pingback: Getting On The Air – Your First Station | Nashua Area Radio Club

  2. Hi, very nice job on the delta loop. Can you tell me what dimensions you used? My 4NEC2 model is giving me lengths at resonance that are significantly longer than 1 wavelength. I’m an experienced modeler, but never modeled a delta loop before and I am surprised by the results. Impedance at resonance (7.15mHz) is 165 Ohms, length of each side 58ft. I was expecting a side length of approx. 47ft. Can’t figure out what’s up with my model. So, I’d like to try modeling your final dimensions if you could please share them with me (including wire gage). Thanks… -Bill W2KWD

    • Hi Bill,

      I don’t remember the final dimensions but a result that is a little larger than 1 wavelength sounds correct. Also, you should cut yours long and plan to trim the final installation to get the match that you want.

      Fred, AB1OC

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