Magic two-element antennas for HF – 6, Double Delta beam podle G3LDO
Honza Bocek OK2BNG provided us with incredible help – he provided us with his articles and took it upon himself to manage a new section on two-element antennas. He suggested contacting Peter Dodd G3LDO and Less Moxon G6XN, who could answer your questions through the forum. we thank you! Contact the authors of the article: Jan Bocek OK2BNG, janbocek@mail.tele2.cz, Jiří Škacha OK1DMU, skachaj@volny.cz
Two-element KV directional antenna systems have attracted the interest of amateurs throughout the last century, and this interest will probably not disappear in the future either. This is evidenced by many lectures, held each year at the Dayton Symposium [47]. As a reminder, we present in fig. 1 shapes and frame dimensions of the most used two-element KV antennas, which have already been described in this series [48-49].
Symbol substitutions used in the text: male lambda – LMBD, big omega – OHHH.
HB9CV, Rudolf Baumgartner designed a supergain phased array system with element spacing 0,125 LMBD. Both elements are full-size with a length close 0,5 LMBD. The advantage is the all-metal design and the possibility to connect any power supply [48].
VK2ABQ, Fred Caton tried to minimize the dimensions while maintaining the electrical properties; he eventually arrived at a square antenna plan 0,25 x 0,25 LMBD. The wire radiators were suspended from bamboo supports, the solution to bending the ends of the elements is important here. From a mechanical point of view, the system was difficult to build, because it manifested itself with less stiffness “rubberiness” systems. The input impedance was high [38].
G6XN, Less Moxon worked with Fred VK2ABQ for several years and the result was an input impedance antenna 50 OOHH rectangle shape. It is very popular in the world under the name Rectangle beam or Moxon beam. The tubular design was described in 3. part of this series [48].
W4RNL, L. B. Cebik is one of the biggest publicists in the field of antennas. For antenna experimenters, his pages are inherently among the favorites. He optimized the Moxon antennas using various antenna programs, reduced the distance between the elements down to 0,14 lambda and optimized the critical binding between element ends [39].
G3LDO, Peter Dodd is another well-known antenna columnist [51-54]. He devotes a considerable part of his work to the issue of small rotary directional antennas for HF. In an attempt to preserve the full length of the elements, he used a geometric shape of the elements similar to a triangle, he therefore named the antenna Double Delta – DD antenna for short. He achieved the reduction of the overall floor plan dimensions by bending the wire ends of the elements towards the mast. The antenna is shown in fig. 1. Its older wire version had a mutual distance between the elements 0,3 LMBD, the version resulting from further development in a tubular design with the extension of the elements with wire conductors already has substantially reduced dimensions – the mutual distance of the elements decreased to 0,16 LMBD. As a result of bending the ends of the elements, the total length of the element is somewhat larger, than a classical dipole. Peter abandoned the earlier effort to preserve the input impedance 50 OHHH; at the mutual distance of the elements 0,16 LMBD achieved the classic average input impedance 28 OHHH, similar to most yagi antennas.
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Description of the Double Delta antenna – DD-beam
The antenna has evolved over the past twenty years; in the following, we will focus only on the design of the basic part of the antenna from metal pipes according to fig. 1b, see also fig. 3. The spar and both elements form “hook”, similar to the constructions of other directional antennas. One element – emitter – we perform divided as a dipole. Peter, G3LDO, although it uses both elements undivided and supplies the radiator with a shunt, but we do not recommend this solution, because measuring and tuning the element into resonance can then be more complicated. Copper insulated conductors are connected to the ends of the tubular elements, which are stretched by a non-conductive cord towards the mast, they roughly follow the course of the pyramid's edges (viz fig. 1a a 1b).
Antenna production
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Tab. 2. Approximate experimentally verified dimensions of the DD-beam antenna, the designation corresponds to fig. 3. The total length of the radiator is based on LZ = 0,576 LMBD = 173/f, total reflector length LR = 0,5875 LMBD = 176,25/f. The dimensions of the gamma adjustment elements are given for R 28 OHHH.
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The design and actual manufacture of the antenna includes a number of elements, topics and possibilities, detailed in previous parts of the series. We will therefore not repeat them in the following text and will only draw attention to some specific moments.
Tab. 3. Measured impedance values for the realized antenna for the band 7 MHz. Frequency given in kHz, R values, X and Z are given in ohms.
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First, we assemble the basic frame of the H-shaped antenna – viz fig. 3. The elements are made of AlMg tubes with a gradually decreasing diameter and are inserted into each other (see past episodes of the series). We connect the conductors electrically and mechanically reliably to the ends of the pipes, preferably by attaching a soldering eye under a screw, typically M6, pulled into the thread, cut into a metal plug, fixed to the end of the pipe. From the point of view of stress on the conductor, it is necessary, so that the tension in the conductor is transferred to both the copper conductor and the insulation. Grimm eyes will work well, which clamp the copper wire and the insulation. Otherwise, we have to lighten the connection points of the conductors to the pipe with an insulating cable. The upper part of the mast above the plane of the tubular parts of the elements is made of a steel tube of such dimensions, so that it can be freely inserted into the main mast tube. Other material can also be used, for example AlMg or laminated bamboo. We will stretch the wire extension on this pipe (D a F). In the F position, the rubber pull-down brackets have proven themselves, used for attachment to car carriers. The actual tube elements of the antenna are bent by the pull of the extension wires stretched towards the mast, and with regard to mechanical strength and stability, it is therefore advisable to use tubes of larger diameters – we recommend at least 20 mm. On sale are pipes with wall thickness 2 mm. Good experience is with averages 20/25/30/35 mm, which can be easily inserted and mechanically combined into one unit. |
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Tab. 4. Cable lengths of transformation l/4 line 50/28 ohm (v m). Cable 75 ohm, k = 0,66.
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The antenna can typically be powered in two ways: An antenna with a divided powered element according to fig. 3 has impedance at the edges of the band around 28 ohms – measured impedance frequency response for the antenna for the band 40 m is given in tab. 3. We will use an impedance transformer for power supply – vf line of electrical length 1/4 LMBD – viz fig. 4. The lengths of the transformation section for cables with a fixed dielectric with a shortening coefficient 0,66 are listed in tab. 4. We create a transformation line by connecting two cable sections in parallel 75 OHHH, so its resulting impedance will be 37,5 OHHH. One end of the line will be connected to the terminals of the split radiator, a standard coaxial cable can be directly connected to the other end of this line 50 OHHH.
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Tab. 5. The dimensions of the gamma adjustment section according to fig. 5
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Another option is, according to the original source, to power the emitter using the gamma section – viz fig. 5. Dimensions for this arrangement listed in tab. 5 are calculated for conversion 28/50 OHHH. The tuning capacitor solution is worth paying attention to, which is formed by the inner conductor with the PE insulation of the RG-213 cable left. This wire is inserted into a tube of internal diameter 8 mm. We measure the capacitance between the conductor and the pipe 200 pF/m. By moving the conductor in the tube, we change the capacity of this capacitor and thus we can compensate the reactive component of the impedance to a minimum value. By setting the dimensions A and L, we are looking for the optimal SWR. We must seal the pipe against moisture.
Antenna settings
First we tune the radiator – a split element of pipes with connected wires. Compared to the dimensions from the table, we first choose the actual total length of approx 10 % larger, so that we have something to abbreviate from. We fix the element on the boom and raise it to a height like this, so that the wires hang freely vertically down and their ends are at least 3 m above the ground. On the conductors you can e.g. mark the length marks with insulating tape, so as not to overshorten the wires in the heat of adjustment. We will look for resonance like this, so that the length of the entire element is close to the value 0,576 LMBD (LMBD corresponds to the center of the band); resonance is usually found at the top of the band or slightly above the band. An SWR meter is sufficient for this measurement. We may not be interested in the absolute value of the SWR for now, which can be (against the normalized value 50 OHHH) and around 1,2, but the position of its minimum and the frequency course.
After fine-tuning the radiator by changing its length, we check it by calculation, the total length seems to match the relationship 0,576 LMBD. Wonderful article (may be caused by different pipe diameters and a different extension cord), we have to adjust the formula for calculating the reflector as well, so that the length of the reflector is proportional to the length of the radiator 0,5875/0,576, so that the reflector is approx 2 % longer. In practice, the length marks on the ends of the wires will serve us well – we mark whole meters and tens of centimeters for the last meter.
The second element, undivided reflector, will therefore be approx 2 % longer than the emitter. We fix the reflector in the correct position on the boom, let the extension wires hang down freely again for now, we raise the antenna to the height again and measure the SWR again. The minimum will be slightly lower in frequency, than in the case of the radiator itself, intrinsic SWR value (against the normalized ones 50 OHHH) will deteriorate to approx 2,3 to 2,5; that's okay though, because the bond between the beam elements is already showing, causing the impedance to drop. If, on the other hand, the SWR was still relatively good, that's about it 1,2, it would mean, that the reflector is long or that the element distance is not 0,17 LMBD. Who has measuring devices RF1, VA1 or MJF-259B, can measure Z and X – DD-beam measurement example for 40 m is in tab. 2. Let's notice, that at antenna resonance the value of Ra is small and the value of Xa is very low. By adjusting the dimensions of the antenna, a zero reactance component can be achieved, but it is laborious and we can only achieve the zeroing of X at a single frequency. In this phase, when we still don't have the extension cords definitively attached, we will therefore settle for values of X not exceeding 20 OOHH on the edges of the band. The data are only indicative, but for practical purposes this approach is quite sufficient.
So we have the antenna with wires hanging freely down roughly tuned, early birders can even make their first QSO. The next step is to adjust the antenna to its final state, i.e. with conductors firmly attached to the mast according to fig. 3. The shape of the antenna allows for a dual arrangement: The wire extension can be fixed upwards above the boom on the extended part of the mast, or downwards towards the mast. The first variant simplifies the issue of rotating the antenna, because above the plane of the boom we can already use a weaker pipe; in the second case, the pipe must be stronger.
First, we tension the extension wires of the emitters with the cords, and for the time being we twist the ends of the reflector wires into a ball, so that the reflector does not affect the measurement. Again, we measure the resonance of the radiator and adjust the length of its wires so, so that the resonance frequency is slightly above the band. Then we stretch the reflector wires to the mast and check the resonant frequency of the entire system, which should already lie in the desired band. At the antenna for 40 m was published e.g. radiator length 24,5 m and the resonance frequency was 7150 kHz. The length of the reflector was then 1.02 times greater, so 24.5×1,02 =25,0 m.
Example experiment for 40 m pásmo
From a disassembled Delta Loop for 15 m remained 4 pieces of trapezoidal elements with a diameter decreasing from 35 on 16 mm, each with a length 5,1 m. The ends of the smaller diameter tubes had an M6 threaded hole to connect the wire chord of the original Delta Loop. For the new beam DD on 40 m these tubes were used without modifications (later it showed during a gale, that at these dimensions there are pipes 16 mm at the limit of usability, the ends of the elements bent slightly at the point of weakness). Two of these pipes were attached in isolation to a plate of insulating material (split radiator), the other two on the aluminum plate (undivided reflector). With four additional clamps, both elements thus created were attached in their center to the boom of the OWA antenna for the band 15 m. For the first attempts, the length of the extension wires was chosen 6,4 m. The antenna was raised 3 m above ground; it resonated slightly below band and had impedance 24 OHHH. For the original OWA antenna, the mast above the rotator was extended by 8 meters for anchoring a long boom 15 m antenna. Therefore, it was chosen to fix the ends of the extension wires of the DD-beam upwards. After fixing the conductors at the top to the mast, the resonance compared to the conductors hanging freely down changed by approx 300 kHz towards higher frequencies, on 7,35 MHz. Therefore, it was necessary to extend the length of each radiator wire to 7,17 m and the length of each reflector conductor na 7,4 meter. The total length of the radiator was then 24,54 m and the length of the reflector 25,0 m. A LMBD/4 transformation line was connected to the radiator (length 7 m), made of two parallel cables 75 OOHH with fixed dielectric and shortening coefficient 0,66, as indicated in fig. 5. The cable has been twisted into a coil shape and thus forms a vf choke. Cable joints must be made carefully and treated with vulcanizing tape against moisture. The final height of the antenna is 20 m above the ground, the tops of the extension wires are at a height of approx 28 m and the average antenna height is LMBD/2.
Operational experience
A dipole was used for comparison 10 m above the ground, oblique dipole z 18 m facing west, Moxon beam fixed in E-W direction 10 m above ground and vertical high 30 m. Opinions on the situation in the zone 40 m and experience are summarized in the section of the series on Rectangle beam for 40 m [48]. It really is a magical area. With DD-beam, you will experience completely different experiences here, than you are used to. Turn signal for 40 m is not such a matter of course, such as. for the band 21 MHz. You will definitely experience pile-ups very soon, and not only from the peripheral parts of the EU, which is quite common, but maybe even an hour-long pile-up from JA, and that's a great experience. You will think, that you are on the bandwagon 15 m.
For example during a very long QSO in the band 7 MHz OK2BNG with JA2DPC, Setsuko asked in great detail about the dimensions of the antenna, because under the brand N8YL and A35PC it used to 40 m Moxonův beam. She only knew the older ones, wired version of DD beam. She sent a long letter, in which she asked for photos. Sam is currently using a rotating dipole on 40 m.
But it is also necessary to admit, that the extra dBs were often not heard when comparing and the differences between antennas can often be understood as subjective. The results of the comparison depend on several factors, e.g. on propagation conditions producing a signal at different angles of incidence, antennas of opposing stations and on their IQ. In all cases, however, the DD-beam is a turn signal, showing distinct directional radiation. If we read, for example. station to an oblique dipole by force S5, i.e. at the noise level 40 m band, then after re-directing the DD-beam the signal is about o 1 With better and therefore more readable and communication is possible. Although the gain of this antenna compared to a dipole is only 3-4 dBd, it is not appropriate to underestimate it. The main advantage is the presence of a lower radiation lobe, which is simply not there at all with other wire machines. The difference in signals can then be around 20 dB and that already means, that we hear or call stations, which we simply do not know about when using the wire. And therein lies the advantage of even a sub-optimal two-element turn signal compared to a straight wire, which does not exactly show significant directional radiation.
The described antenna is structurally simpler than HexBeam, described in the last part of the series (but its advantages are again the attractive value of the input impedance 50 OOHH and slightly better profit). A DD-beam can be made in one to two weekends with just basic workshop equipment, without further help and with a bit of luck.
So what is the strength of this antenna? Let's look again at fig. 1 and we will see, that in dimensions and then in radiation resistance. DD-beam with dimensions corresponding to a classic antenna on 15 m is functional in the band 40 m. It is unbelievable, but when the OWA pro antenna was finished and working well 21 MHz used as the design basis for the DD-beam for the band 7 MHz, it was black and white. Ready antenna DD for 15 m has dimensions corresponding to a classic antenna for the band 6 m.
Other types of two-element antennas have approximately similar electrical and radiation parameters. From a theoretical point of view, there is of course a lot of room for various discussions, but practice tends to be more merciful. A two-element model will remain a two-element model, although phased systems (HB9CV) or “detention” systems (G6XN) they have their pluses. In the end, the common prerequisites are mainly a solid mechanical design and the possibility of somehow getting the antenna into space.
The DD antenna is taught to all experimenters, who for various reasons do not buy commercial antennas, but allows the construction of those as well, in which for various reasons “normal” the dimensions simply do not fit. For example, a direction indicator for a band 20 m is as big as a barrack; The DD antenna is four times smaller. And that's already worth the experiment.
What to say at the end of the series?
A total of six articles in our series were devoted to simple directional antennas for KV bands. The main reasons and our internal arguments for it, that we took the time, energy and work, associated with concentration, by checking essential information and finally by writing for interested parties and readers, they are based on two basic statements:
1. Even the simplest directional antenna, especially if it is rotatable and placed at a suitable height, opens up new horizons for amateur operation and moves the station into a completely different technical and operational category. It makes available the area of the world of communication, often unavailable when using improvised or otherwise rigged antennas, especially “somehow cut long wires”, brings previously unknown experiences and connections with stations, which would otherwise not have happened.
2. Complicated, heavy, dimensional, optimized and professionally manufactured directional antenna systems requiring massive masts, powerful rotators, large plot of land, intensive inspection and maintenance, insurance (remember this year's storms) apod. they can also have very good electrical and communication parameters when placed appropriately; but they usually also have an appropriate price. However, they also have the prerequisites to move up the ranking of stations, who do not abound in said possibilities or who derive satisfaction from it, when equipped with a self-made directional antenna. Modern two-element directional systems, especially in a design with suitably reduced dimensions, for this they offer rich and interesting possibilities, especially today, when it is no longer a big problem to play with computer models or measure relatively delicate electrical parameters with generally available antenna analyzers. It was not intended to dismiss wire antennas. For LBDXing, they are hard to beat when placed high enough. They show radiating lobes, which can illuminate the Earth from BY to KW6 better, than DD-beam. But that would be about something completely different.
In the end, perhaps just a modest wish: It is clear, that there are operators equipped with a wealth of knowledge, rich experience and rich possibilities of implementation or use of high-performance antenna systems. For you, the information from the series may not have been very interesting. But there are also many others, which individual parts could be a source of inspiration, provide topics for searching for the necessary information and encourage the determination to do something to improve their equipment so, to better apply themselves on amateur bands. Believe, that the peace of mind over mastered problems is really worth all the effort. We wish you the best of luck, success and satisfaction to all.
Literature
[47] www.cebik.com/FDIM, Dayton 2002
]48] Jan Bocek, Jiří Škach, Magic two-element directional antennas for KV – 3, RA 3/2002
[49] Jan Bocek, Jiří Škach, Magic two-element directional antennas for KV – 4, RA 3/2002
[50] http://www.cebik.com
[51] Peter Dodd,G3LDO, Wire Beam antennas and the evolution of the G3LDO Double-D, RadCom, 6/7 – 1980
[52] Peter Dodd,G3LDO, Further Evolution of the G3LDO Double-D antenna, RadCom, 4/1990
[53] Peter Dodd, G3LDO, The Antenna Experimenters Guide, RSGB 1991,1996
[54] Peter Dodd, G3LDO, Backyard Antennas, RSGB 2000, 2002
[55] Peter Dodd, G3LDO, Review of the MQ2 Mini-Beam Antenna, Practical Wireless, 8/1999
