Three Element Loop Parasitic Arrays
In previous articles in this series we noted the advantages of the ADR or asymmetrical double rectangle as a single element and in two element loop parastic arrays.[1] The major advantage of this type of antenna over that of the simple cubical quad is a significant widening of the bandwidth (BW) for both front to back ratio (f/b) and SWR. An additional advantage is the fact that the change in gain over the full BW is smaller than the quad.
In this article, I shall examine three types of three-element arrays which involve using the ADR as the driven element. The arrays to be discussed are: a pure ADR array – Figure 1, and two types of hybrids – one with a ADR driver and simple quad loops for the reflector and director – Figure 2 – and another which is a quagi with a ADR driver/reflector and a rod or dipole element as the director – Figure 3. The 3-element quad is in Figure 4.
The Antennas
The first loop/rod hybrid antenna was the quagi which was conceived by Wayne Overbeck, N6NB, in the 1970s. He used two loops as a driven/reflector pair with rod elements as the directors. The major reason behind this antenna was the fact that large multi-element Yagis at VHF and higher exhibited very low feedpoint resistances (Rin) with the attendant matching and loss problems.
The quad driven/reflector pair raised the Rin to make matching easier. Along the way he found that he could get more gain out of the antennas if he used a trigonal reflector but abandoned this because of its complexity. The final antennas that he settled on therefore had the virtues of both simplicity and feedability and an insignificant gain advantage over pure Yagis. He has told me that f/b was not a consideration in their design.
We shall start, as always, by examining the performance of our “reference” 3-element quad. Our design criteria are: the highest gain compatible with a SWR < 2 and the highest possible f/b at either extreme of our desired BW. I shall not be listing the SWR data since this was found to be the limiting factor with all of the antennas discussed here. By this I mean that the SWR limits were reached long before the f/bs sank too low. All of the antennas are at the SWR limit at each end of the band.
Three Element Quad
This is the simplest “all loop” antenna and is seen in Figure 4. Table 1 gives us its dimensional and performance characteristics.
This is a good antenna with a gain of a decent 4-5 element Yagi. Unlike the 2-element loop antennas discussed in a past issue of antenneX, this 3 element antenna and all of the others we will be discussing here exhibit really “clean” patterns to the rear. Their f/r or front-to-rear ratios are as good as their f/bs.
Rin is a little low however but not low enough to cause feeding problems. Also, the SWR pattern is not symmetrical about 146 MHz. It is skewed slightly in frequency since, to get an SWR < 2 at 144 and 148 MHz the antenna had to be tuned to 146.1 MHz.
Antenna Comparisons
Now that I have established the basic performance of our reference antenna I would like to go to the gain – Figure 5 – and f/b – Figure 6 – comparisons of all four types.
Gain
Figure 5 compares the gain of the four over the desired BW of 144-148 MHz. The curves are almost parallel with the quad which has the lowest gain. The ADR array has the highest and the two hybrids fall in between. The difference between the quad and ADR array is about 0.6 dB across the board.
F/b
The f/b figures in Figure 6 present more of a mixed picture. The ADR clearly stands out since it is the only one having a f/b > 20 across the whole spectrum. You may note that the peak f/b at the center frequency is not as high as we would expect from studying the 2 element loops. The reason for this is that I had to detune the parasitic elements in order to get acceptable performance at the band edges. However the usefulness of parasitic antennas is dependent on how they perform over a desired BW and not at a single frequency.
Let us now examine each of the ADR derived antennas.
Three Element ADR
In order to understand the ADR array we should first look at an individual element – seen in Figure 7. This antenna is shown with its feedpoint up instead of down, but no matter. The primary loop is the larger loop and the secondary loop is the smaller. I empirically found that the best ratio for individual loop sizes involved setting the center wire at 40% of the overall height. With all of these antennas the overall loop height of the driven element is 0.4 wl. Therefore the center wire splits the antenna into two loops with the primary being 0.24 wl and the secondary being 0.16 wl high. In my ADR shorthand we could characterize the driven element as one which is .4/.24/.16 wl for overall height/primary loop height/secondary loop height.
I now made the reflector 10% taller and the director 10% shorter. Again, I maintained the center wires at 40% of the overall height. Therefore the reflector is .44/.264/.176 wl tall while the director is .36/.216/.144. These dimensional relationships seem to line up the 3 radiators of all the elements quite nicely although I have no idea at the present time whether this makes a difference in their performance.
Table 2 gives us the dimensional details and performance characteristics of the 3 el ADR.
The “pure ADR” parasitic array exhibits higher gain, a wider f/b BW and a higher Rin than the 3-element quad. Interestingly, the boom length is shorter. This is an excellent antenna having the BW and gain of a OWA (optimum wideband array) Yagi of 6-7 elements. The Rin provides a good match to 50-ohm line.
If one detunes the antenna and accepts a slightly lower gain, the BW is fabulous. It can be easily extended to cover 8% with a gain at the center of about 9.4 dBi. An antenna such as this can easily cover the full extent of the 70 cm band (420-450 MHz or almost 7% BW) or, with 1″ tubing elements, the entire 6-meter band (50-54 MHz).
My understanding of BW requirements, from a query to Dean Straw, N6BV, is that most communication at VHF/UHF is highly specialized in that there are quite narrow band segments used by those playing with EME, meteor scatter etc. He said that the only band, from a ham radio viewpoint, where a really wide BW is necessary is 70 cm. So, from a practical point of view, we can push the gain higher on virtually all of the bands and still have an acceptable BW. However, there are commercial and other non-amateur services for which such wide BW is necessary.
The Hybrids
The data for a 3-element quad/ADR hybrid are in Table 3 and that for the ADR/quagi in Table 4.
Please note that the height of the driven element of the quad/ADR hybrid is exactly the same as the ADR array’s and the height of the reflector/driven pair of the ADR/quagi is identical to that of the ADR array.
Note that the element spacings are quite different from each other and from either the quad or ADR arrays. The center frequency has also shifted up in order to make sure that the SWR limits are not exceeded. Both antennas will match a 50-ohm line. The peak f/b of the quagi with the rod director is not as good as with the antennas with loop directors.
Conclusion
It is hard to say which antenna would be preferable to a constructor/experimenter. There are always tradeoffs in antenna design-one always has to balance gain, BW, Rin and ease of construction to choose one design over others. The all-ADR array is the best of the class and relatively easy to construct using the same techniques worked out with quads.
While the gain difference of 0.6 dB between the 3 element quad and ADR antennas might seem inconsequential you might be surprised at the cost you would have to pay to equalize the gains. To get another 1dB or so out of a quad you would have to construct one with 4 elements on a boom 50% longer than the one we have discussed. Gain comes at a price – taller, slightly more complex elements versus more of them on longer booms.
To equal the performance of any of these antennas one would have to construct Yagis with many more elements on far longer booms. From this vantage point the antennas examined here are certainly more “compact” antennas than the Yagis albeit with more cubic volume.
The extrapolated performance figures for a 3-element ADR at 28.5 MHz and at a height of 2 wl above ground are: boom length of 3.57 meters (11.7′) with a gain at the first elevation lobe of 15.5 dBi. A 20-meter antenna with the same gain would have a boom length of only 7.19 meters (23.6′). You would need a really long multi-element Yagi to get this kind of performance.
The challenge at HF is to see how these antennas can be designed with thin wires since 3″ or 75 mm diameter tubing for the wires is out of the question. What we will have to do is apply L.B. Cebik’s techniques of substituting two thin wires to duplicate the performance of one thick one. His quad designs are promising and there is no reason why they can’t be applied to any other type of loop. To this end I am experimenting with a single element ADR which I hope will cover all of 21-30 MHz. If this plays as expected I will publish the results here and we will then have the ability to apply the lessons of multi-element antennas such as the ones described here to HF.
I would like to emphasize that these ADR arrays are not pioneering. I have just searched for and found many Japanese websites where Hentenna 2 and 3 element parasitic arrays are popular for 6 and 2 meters. The basic difference between the Hentenna and the ADR is that the former is just one variant of the latter. The Hentenna involves fixed dimensions to enable 50 ohm feed at the middle wire. Therefore its flexibility is limited and there is some sacrifice in gain when compared to far end wire feed. Nevertheless, it is a proven and popular design in Japan and the Far East and, after almost 30 years, I am puzzled by its lack of popularity in the rest of the world.
Construction Dimensions for 146 MHz
Dimensions in meters for all four antennas discussed in the text: ADR elements are highlighted
Originally posted on the AntennaX Online Magazine by Dan Handelsman, N2DT
Last Updated : 2nd June 2024