Delta-loop antenna 80m


Legends, theory and practice

In the following, the construction of a simple multiband delta-loop antenna for the bands 80-10m is described. With the antenna calculation program EZNEC, the setup is theoretically prepared and documented with the inexpensive antenna analyzer miniVNA or graphically documented. Likewise, the advantages of a coaxial cable compared to a chicken ladder with Matchbox for this type of antenna are practically and metrologically documented.

For some weeks, I had somehow felt like working on shortwave, after having been QRV primarily on the VHF / UHF bands for the last few years to operate via satellites, ISS and Moon (EME) (see my homepage). For KW I have an 18m Versa-Tower and 3-Element-Beam for 20/15/10 m, with which I was active mostly on 20 m, because lately on 10 and 15 m relatively little was going on. I had to realize that after 19.00h also on 20m quite quiet prevailed. At 40 and 80 m was unfortunately nothing for me, because my half dipoles for the two bands had been demolished for several years, and these were not lacking because of my VHF / UHF or space activities anyway.

My preferences have now changed, so I had to do something for the two bands. Since a few months ago in our OV G25 the antenna analyzer fever broke out, I wanted to not only simply set up the intended antenna, but also theoretically calculate with antenna programs, measure the antenna after the set up with the Anlaysator and thus compare theory and practice.

Theoretical considerations and preparations

After we had the Delta loop of our OV home G25 back in the gears (see action Nobel laureates …) and I was very pleased with the send / receive properties, I wanted to build this type of antenna at home. First of all, I made myself smart in Rothammel, ARRL Handbook and Internet. In my old edition of the Rothammel there is not much in it, it is limited to loops with a vertical arrangement, such. Cubical quads. On the internet I got some tips, partly with contradictory legends, which made me curious. Finally, I found the best description in the ARRL Handbook.

In the Handbook, the antenna is referred to as a Loop Skywire, and presented by the radio operator community as a completely undervalued secret tip in terms of cost / benefit ratio. I had to agree fully later. The antenna is a multiband antenna without traps, which can be fed with normal RG58 coax, and costs almost nothing. It is resonant at the fundamental frequency 3.5 MHz and at any multiple, including 40, 30, 20, 15 and 10 m. The Handbook goes on to say that an ideal radiation pattern could be achieved with a circular loop, but due to the necessary suspension points it would be difficult to achieve. As a compromise would then be a square arrangement in question, which was also described here. Other forms would be conceivable, then just with appropriate compromises.

The next step was a site survey through my garden. Since the property boundaries are covered by tall trees, a square loop, as described in the handbook, was out of the question for me, but only a triangular arrangement. Due to the possible suspension points, a triangle of about 20x30x30m would be suitable, fed by a coaxial cable in one corner of the triangle, i. at the suspension point Versa Tower. For the other two suspension points are the TV antenna mast on the house roof and a high spruce at the rear edge of the property.

Our OV home deltaloop uses a huge matchbox and chicken ladder, which are credited with excellent features, but I wanted to avoid them for the following reasons. Due to the metal Versa-Towers house wall-carrying a chicken ladder would be difficult to realize, and a thick Matchbox I have not. So for me only coaxial cable as feed in question, as suggested in the handbook. As a comparison, I have shown the results of the Deltaloop with chicken ladder below, with very interesting results and conclusions.

Next, the total length of the wire was to be determined. For this I downloaded the antenna calculation program EZNEC from the internet. Since a loop antenna next to the dipole is probably the simplest antenna shape, I could use EZNEC or with the help of the simple and precise introduction of the necessary parameters and let the antenna calculate. Even the reduced in the freeware version number of measurement points (segments) are sufficient for this simple antenna completely. EZNEC also takes different soil conditions, construction height and wire gauge into account, so that I trust the program more than the usual simple calculation formulas c, f and k.

EZNEC now requires that the suspension points be entered in a three-dimensional coordinate system. In terms of my property, the Y-axis is a parallel line to my eastern property boundary and the X-axis to the northern boundary. The Z axis denotes the height of the suspension points, in my case this is both my Versa Tower as a starting point and feed. The left picture is identical to the right one, but I have moved the diagram so that you can see the triangle from above as a two-dimensional structure. Moving to any angle is easy with EZNEC.

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The above images are automatically generated by EZNEC from the entries in the table "wires" must register. Here I have divided the two long wires again into 2 parts, because I wanted to simulate the intentional sagging of the long wires here. So I split the thing into 5 parts, although of course it consists of a single wire.
The part wire no. 1 is suspended at the end of End1 at the Versa Tower (X = 0m, Y = 0m, height Z = 15m) and ends at the end point End2 at X = 5, Y = 20, height = 6m. End2 of sub-wire 1 is identical to End1 of sub-wire 2, etc. In the column Conn, the connection points automatically enter, e.g. W5E2 into column 1, i. so wire1 / end1 meets wire5 / end2.

D

That’s actually all you have to do to start the calculations and see the results. With the SWR button you can now display the resonances of the antenna structure over any frequency range, as shown below.

The diagram nicely shows the resonances at 3.6 MHz and their multiples. Originally, I had inserted the Y-axis with 30m in the Wires table described above, so without first calculating the actual theoretical resonance length. As it turned out, I was not so far off with my guessed values. I only had to add 1m to Y, and I was at the desired resonant frequency according to the SWR diagram. The actual length of the individual wire segments and thus the total length I could in the o.a. Read the coordinate image if I moved the mouse pointer over the respective segment. That was then 86 m.

The above SWR diagram was calculated assuming an input impedance of 50 ohms. At 3.6 MHz I actually have excellent values ​​with 56 ohms and SWR 1: 1.5. At 7 MHz I have theoretical 128 ohms and SWR of 1: 2.8. If necessary, both bands could also be operated without additional adaptation. I then repeated the same measurement assuming an input impedance of 200 ohms, hoping that the upper band SWR values ​​would improve, i.a. diagram.

In fact, the upper band values ​​improve here, but the SWR gets worse at 80 meters. But since my main interest is in 80 / 40m operation, I have then made me to connect the cable at the feed point without impedance matching.

The diagram indicates a high narrow band of the two wish bands. In the following picture the SWR can be seen again for the range 3-4 MHz in high resolution. Due to the high resolution, even the theoretical resonance value or SWR of 1: 1 with 50.4 ohms is now reached. The bandwidth at SWR

The 40m band looks similar, but with a worse but acceptable SWR without impedance matching:

Below are the EZNEC calculated directivity diagrams for all shortwave bands. At the relatively low antenna height, the antenna at 80m is naturally a steep spot in the direction of 90 degrees upwards, at 45 degrees elevation it has 3 dB less. Overall, however, it acts in the horizontal almost like an omnidirectional, the maximum difference north / south (about Y-axis) to West / East Ca. X axis) is about 3 dB. In the higher bands, the directional characteristic and Verzifpelung correspondingly more pronounced.

Directional diagram at 80m

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For the sake of completeness, the plots for bands 40, 30, 20 and 10m follow, which already show a clear directional characteristic. As expected, the antenna radiates flatter with increasing frequency.

Directional diagram at 40m

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Directional diagram at 17m

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Directional diagram at 20m

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Directional diagram at 10m

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And now the practice

After getting fit in theory, I started building the antenna. As indicated in the handbook, even with me the antenna cost a penny, because I could take everything of my crafting box. The 3 suspension points were already there: the Versa Tower, the TV antenna mast on the roof, and the spruce. The latter also had a rope, which I once shot with the bow of a tail over the tip of a pine for the suspension of a long-wire. 1: 1 balun, wire strand, insulators for the bracing and rope clamps I had too. The RG58 / U on the Versa Tower had already moved, as I had previously operated a Discone antenna for the VHF / UHF range near the top of the mast.

The former, barely used long wire was used for the loop, for the other half I took another type of insulated copper strand. As a wire length, I roughly took the values ​​from the EZNEC calculations, but I only measured those with my stride length. A strong step with my short legs = 1 m. Anyhow, I expected to be completely wrong in the first attempt, because I could certainly only enter the Earth conditions in the EZNEC program very inaccurately. It would have to be over the thumb so somewhere between 80 and 90 meters.

My Versa Tower is firmly screwed to the garage wall, I can crank it down to the height of about 7-8 m. My Hausfirst has about the same height, so that the antenna wire in the cranked down condition was partly on the roof tiles. At the spruce, I pulled the wire to target height. Before I started cranking up the mast, which after all demands a pretty athletic performance from me old boys, I already wanted to do the first measurement. On time Heinz, DL9NDG, appeared to me with the antenna analyzer minVNA, which he had borrowed shortly before from the other Heinz, DD9KA. I quickly found a suitable BNC adapter on PL, and off we went.

The result is to be admired in picture below. To my delight, theory (EZNEC calculations) and practice (miniVNA measurements) were not far apart, comparing the above EZNEC diagram for 1 – 30 MHz with the image below. The same resonance points are displayed in both diagrams! Also, the higher resolution at 3.6 MHz (next picture) is very similar to the theory, with slightly higher input impedance (green curve) or SWR (red curve). Even the resonance point was exactly at 3.6 MHz, so I randomly sized the antenna wire randomly from the start. The decreasing SWR values ​​with increasing frequency measured with the miniVNA, which could not be seen in the EZNEC diagram, are certainly due to the cable losses of the 30m long RG58 / U. With EZNEC I had calculated the values ​​directly at the feed-in point of the antenna.

Heinz I and II left the antenna analyzer to me and I was able to keep it quiet. Then I cranked the mast at nominal height about 16 m high, and performed a new measurement. As feared, unfortunately, the previously perfect resonance point moved a bit, to 3.7 MHz, see Dieagrams below. To my delight, however, the SWR improved to an acceptable 1: 1.7 at 40m.

For 80 m, the shift of the resonance is still ok, but for the higher bands, the resonance frequency wandered outside of the amateur radio ranges. At 40 m she was then shifted from the perfect 7.05 to 7.28. On the other hand, the SWR, to my delight at 40m, improved to an acceptable 1: 1.7.
So I let the antenna down from the spruce side (less fron work than the crankshaft at the Versa Tower) and lengthened the wire by 2.70. This length I had in wise foresight already off, i. I had a bit diverted as a possible reserve. Then the spruce side was raised again and a new measurement was made. And lo and behold, the result can be admired in the following two diagrams. Here, the red curve is the SWR, the green curve is the impedance. The exact values ​​for the best SWR and other interesting values ​​can be read in the lines above the diagram.

SWR and impedances in the frequency range 1 – 31 MHz

SWR and impedances in the 80m band

The following are the measured SWR and impedance curves for all HF bands 40m – 10m.

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