One of the hams on my twitter feed, Dan YO3IBW posed a question asking if he should run a delta loop for 40M. Dan’s antennas are situated on the top of his apartment building in Bucharest. The base of his mast is situated at about 30M above street level, with antenna firing E/W. I decided I would model several antenna scenarios to help illustrate Dan’s options.
I am a huge fan of delta loops having had one for 20M in my college days. They are inherently quiet, exhibit gain over a dipole, are easy to construct and are somewhat broad-banded. There are essentially two modes a simple delta loop can operate in: Horizontally-polarized and thus like a dipole, and vertically-polarized and thus like a vertical. I have omitted discussion of the horizontally-mounted delta, where each of the three vertices of the antenna are at the same height – this is another type of antenna entirely not unlike a rhombic antenna.
My previous configuration was fed as a SCV, and it was the best vertical antenna I have ever used. I compared to an Cushcraft R5 and a Hustler BTV-5, both of which were never used once I had installed the delta loop.
40M Delta Loop Model
The illustration above is a very simple delta loop, equilateral triangle with feed point at the bottom. Other feed point options are at the top (for horizontal polarization along with bottom), and side or corner-fed for vertical polarization. The loop is made from one-wavelength of wire at the design frequency of 7.1, or 42.2M. Each side is ~14M with the vertex of the antenna about 7M higher than the base.
In horizontal-polarization mode, either top of bottom-fed at the height of 30M above the street, this antenna shows a classic figure-8 of a dipole and has a very low take-off angle.
40M Delta Loop, Horizontally Fed at 30M High
Note the oblique angle is approximately the same as the wire illustration above. The Y-axis is pointing North, the X-axis is pointing East. The maximum gain is broadside to the wire, essentially E/W, with a take-off angle of about 10 – 20 degrees. At the elevation angle of 15 degrees (FB for DX), the azimuthal pattern is a classic “Figure 8″.
40M Delta Loop, Horizontally Polarized at 30M High
Of course, the blob-ish top hat of RF radiating at a very high angle cannot be avoided due to the height above ground of 3⁄4 wavelengths. The end result of this high-angle blob will be utility as NVIS (only when ionospheric FOT will allow) and possibly a bit more common-mode noise. Neither of these situations would critically affect the low-angle DX performance of the antenna.
If the delta loop were fed on the side or corner, the angle of radiation is typically at low angles. In this mode the antenna acts more like a self-contained vertical antenna. L. B. Cebik W4RNL left us a legacy in his work around SCVs and should be referenced to learn more about SCV and antennas and modeling in general.
40M Delta Loop, Vertically Polarized at 30M High
A corner or side-fed delta exhibits good low take-off angles in general. I this case, due the height above ground, the primary lobe is actually at 10 degrees and a secondary lobe is at 40 degrees. The 3dB vertical beamwidth is actually 47 degrees, making the take-off angles of this antenna useful for DX and some medium range communications.
If this antenna were actually less than 1⁄2 wavelength above ground, you would see a more traditional vertical take off pattern. A general rule of thumb is to mount vertical antennas many multiple wavelengths or less than 1⁄2 wavelength above ground. Many wavelengths above ground produce many well-formed lobes of radiation. Less than 1⁄2 wavelength above ground produces one well-formed lobe of low-angle radiation. At 3⁄4 wavelength above ground, this “vertical” has indeterminate formulation of lobes but still a useful vertical pattern in my opinion.
The azimuthal model is more omnidirectional than the bottom-fed delta loop.
40M Delta Loop, Vertically Polarized 30M High — 15 Degree Azimuth
As vertical antennas go, this is a good performer. Consider how the least favored azimuth is still equal to the best gain of an average vertical antenna. I hope to have a comparison of vertical vs. SCV Delta Loop posted in the near future. As mentioned before, the vertically-polarized Delta is the best vertical antenna I have used.
Antenna models are most useful when used in comparative analysis. I have superimposed four different Delta Loop models to evaluate their vertical take-off patterns.
Comparative Evaluation of Horizontally and Vertically Polarized Delta Loops
The elevation plot shows the four different models of Delta Loop with patterns oriented broadside (E/W) at 30M above ground. The gain scales have been normalized for direct comparisons:
- Black is a top-fed Delta Loop. Polarization is horizontal.
- Blue is a bottom-fed Delta Loop, fed in center of bottom wire. Polarization is horizontal.
- Green is a corner-fed Delta Loop. Polarization is vertical.
- Pink is a side-fed Delta Loop, fed 25% up the length of one side. Polarization is vertical.
The models show the horizontally polarized antennas exhibit 5-6dB over the vertically polarized antennas at 15 degree take-off angle. 6dB is a full S-unit, and when signals are weak this can mean the difference between noise or readable S1 signals. In this case, the bottom-fed loop is the superior DX antenna.
By the way, there is little gain difference between top and bottom-fed Delta Loops. Save yourself the extra coax and feed from the bottom instead of top.
There is a deep null in the azimuthal projections to the N/S with the horizontally polarized loops, thus the side– or corner-fed loops would be much better performers favoring these angles (see previous azimuth plots). This might mean the difference between the other person hearing you or not at these compass points.
As to which is the best antenna, I can argue the best-behaved antenna will be the bottom-fed (horizontally polarized) Delta Loop. From Romania, this antenna would favor Asia/Oceania and Central/South-America. Of course, I am sure it would also do well into the Eastern Seaboard of the United States. This antenna will not at all favor Canada/Western United States nor Africa.
In support for the vertically-polarized Delta, it would seem a better all-around performer albeit an S-Unit down E/W from the bottom-fed loop. To the E/W it will perform with a few dB gain over a traditional vertical, and will also provide equivalent performance to a vertical shooting N/S.
The ability to switch feed point locations is desirable. Automatic switching of the feed point can be a bit of a design challenge (one I have been considering for about a year). If ready access to the base of the antenna is convenient, a manual compromise might be easy to implement. If you were to feed the antenna using coaxial cable, two coaxial feed point insulators might be installed in the bottom wire: One at the mid-point of the wire, and one at the corner. One of the coaxial feed-point insulators would be shorted using a jumper (or coaxial plug wired appropriately) while other the feed-point is fed.
To conclude, the better antenna at this height is the bottom-fed Delta Loop. Vertically polarized antennas at this height above ground exhibit an “in-between” vertical take-off angle, either favoring much higher or lower placement of the antenna system. As a horizontally antenna is higher above ground, well-formed lobes of radiation occurs. The well-formed lobes, however, can be problematic if one desires reception in non-favored directions: Well-formed lobes also mean well-formed nulls! In this case, the side– or corner-fed Delta Loop is more versatile at the expense of an S-unit broadside. Obviously a means to switch the feed point to switch polarization provides the best utility.
In my experience, a Delta Loop is a fantastic performing antenna. I have a couple of corollaries based upon my experience and supported by modeling.
- If you have space for an Inverted V antenna, a Delta Loop antenna at the same height is preferred (if there are no physical obstacles)
- A side-fed Delta Loop is a clearly superior to a traditional Vertical antenna.
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