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K5K Kingman Reef DXpedition October 2000: 80,841 QSO's in 9 days, 14 hours, 20 minutes SVDA Switchable Vertical Dipole Arrays |
The recent DXpedition to Kingman Reef in October, 2000 utilized vertical arrays on the 20-17-15-12-10 meter bands. The basic design is the product of several contest DXpeditions by "Team Vertical", operating as 4M7X, 6Y2A, 4M1X and 6Y4A. The 6Y2A multi-multi operation set a new CW world record in the 1998 CQWW DX Competition. What made this achievement note-worthy is that it was done from a 2-point (North America) location. The 6Y2A story and additional DXpedition information is available at http://pages.prodigy.net/k2kw . The 4M7X operation utilized more sophisticated vertical arrays, featuring 11 elements on each of the 20-15-10 meter band arrays. The final score for 4M7X was short of expectations, due primarily to severe generator problems (off the air for 6 hours) and horrible weather conditions that had ocean debris rolling through the antenna field, taking down all (43) antennas several times - except the 160 verticals (thankfully). The storm hit the Venezuelan coast three days later with full fury, causing massive mud slides and loss of life.
Kenny, K2KW, is the DXpedition coordinator for Force 12, Inc. and wrote an overview comparing verticals and Yagis on DXpeditions (available at http://pages.prodigy.net/k2kw/dxcomp.htm). Kenny's letter communicated to the Kingman Team the excellent and unsurpassed performance properties of verticals at their destination. This, plus the history of Team Vertical convinced the Kingman Team that verticals were the correct selection. Some team members remained skeptical, as verticals on salt water performance is something that needs to be experienced to comprehend. The Kingman Reef DXpedition was an excellent opportunity for others to share the experience. Comparing raw gain figures between two antennas (i.e. Yagi at 30' and 2 element vertical) is not the main issue, it is the energy from each antenna at low angles. Eliminating one hop is worth a good 10dB and the lower angle aspect (transmit and receive) might omit more than one hop. Add to that the difference in gain at the low angles. Having a single lobe with energy down to 1° is the key to DX performance. After 9days 14 hours 20 minutes and 81,000 QSO's later, the K5K team members were convinced!
Taking people and equipment to a remote, uninhabited location is a serious proposition. Safety is the top priority. To underwrite safety while providing the highest performance, the antennas needed to be lightweight, plus being quick to erect and take down. Keeping them simple was paramount. The reef is only 25' across and 40' at the widest part, so radials were an issue, as well as how to guy anything on the coral and shell rubble. After several discussions with Bob, K4UEE, it was decided that the most basic vertical array would be used on all five (5) bands, 20-10 meters: 2 element, parasitic vertical dipoles. The resulting design is the SVDA, which eliminated radials, reduced installation and take-down time, provided a smaller footprint (very limited real estate on Kingman Reef) and slightly increased the gain over a 1/4 wave vertical system. On the reef, the time required to take down, clean and pack all antennas was only a portion of the four hours for the entire set-up.
Most amateurs are familiar with phased verticals, such as the popular 4-square. This is a fine array, using four verticals (usually 1/4 wavelength high with radials) providing remote, switchable directivity covering 360° and a front to back ratio (F/B) in the 15-20dB range. Unfortunately, it is complex and not suited to Kingman Reef. Multi-element parasitic antennas are normally only seen as Yagis or wire arrays. There is no reason not to use parasitic designs for verticals and Team Vertical has used them for several years. The most performance for the weight and using minimal space is the 2 element parasitic vertical dipole design. The vertical dipole has no radials and has about 1dB more gain over a comparable 1/4 wave, due to compression of the lobe in the vertical plane.
The target zones from Kingman were in two directions, north and south; perfect for a 2 element design. The 1/2-power beamwidth (where the lobe is down 3dB from maximum) is about 140 degrees. Making it switchable to the opposite direction covered 280 degrees on the main lobes. The plots are shown below. The F/B was kept at 10dB on the model, which proved to be the minimum observed in real life. How to switch directions on the design was a challenge.
Salt water is terrific for antenna efficiency, but terrible on antennas and equipment. The salt spray can cover everything and cause problems on antennas where the high voltages will arc and burn up portions of guy lines and insulators. Having been through this many times in the Caribbean enabled the right design to be developed. The vertical dipole has its feedpoint higher (1/4 wave above ground), so there is a bit less salt spray on it than at ground level. The high voltage point for potential arcing is only one spot: the bottom insulator, about 1 1/2' above ground. There are only 2 connections: the feedpoint (just like a horizontal dipole). The guy attachment point is just above the feedpoint, which is high current and low voltage. This combination of features makes the vertical dipole easier to maintain than 1/4 wave with radial connections.
The feedpoint impedance of a single vertical dipole is about 90 ohms. If fed directly, this is a VSWR of 1.8:1 at the antenna, which is not a problem, as long as the equipment can load into it. This is an attribute that makes the vertical dipole excellent for arrays. Most are aware that a horizontal dipole is about 70 ohms. When parasitic elements are added, the feedpoint drops and can be as low as 10 ohms, depending on the design. This is why most Yagis use a matching device to transform the feedpoint impedance up to match the 50 ohm coax. Having designed and built direct-feed 50 ohm Yagis for close to 10 years (e.g. Force 12's Magnum 620, 610, 810 and all 6 & 2 meter Yagis), the advantages were desired for these vertical arrays. The 50 ohm feedpoint requires no match, which is one less part for a rough environment like Kingman Reef, and the arrays are broadbanded. Selecting the right design for the 2 element vertical dipole array lowered the feedpoint to 50 ohms. Nice.
Switching directions was the toughest challenge. To keep things simple, there would be no relays, no wires and time to change needed to be less than 30 seconds at the antenna. The driven element (driver) was to remain untouched during direction changes, so the parasitic element was the only position available for changing. This reduces the possibility of making a mistake when people are tired, or in bad weather. The design selected uses a driver-reflector for the northern direction and a driver-director for the southern direction. The basic design is shown below:
Basic SVDA Design |
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The driver remains constant and the parasitic element is switched from a reflector to a director using the manual switch towards the bottom end of the element. Besides the base insulator, as on the driver between the bottom of the element and the base post, the parasitic element has an additional insulator part way up from the base insulator. The switch is placed across this insulator. When the switch is open, only the upper portion of the parasitic element is active, functioning as a director. When the switch is closed, the lower part (tubing) between the two insulators is added to the length of the parasitic and it now functions as a reflector. |
A 2 Element Vertical Dipole Array (not switchable) at 6Y2A in Jamaica, 1998. A white Force 12 B-1 balun and white coax shows the feed system. The white coax goes to the stick and then it is black coax. This is because the white coax is a 1/4 wavelength of 75 ohm coax, transforming the 50 ohm feedpoint up to 100 ohms. The black coax is 50 ohm and goes to a Tee fitting that combines a companion 2 element Vertical Dipole Array aimed the same direction (off to the right of the photo). The total antenna is called a "2 X 2", or "2 by 2", and was designed by Kenny, K2KW and Tom, N6BT for Team Vertical. It provides about 4dB additional gain over a single 2 element array, has a narrow beamwidth and deep side nulls of >20dB. It is aimed directly at central Europe. This is the north coast of Jamaica and the wind and water are rarely this calm. At times, the bases of these antennas can be under water. |
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Driver-Reflector Direction |
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Azimuth (looking downward from above array) Overlay plot of both directions. |
Driver-Director Direction |
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Overlay of both plots in elevation. Driver-Reflector pattern is to the right. Driver-Director pattern is to the left. |
The above show why the SVDA is such an excellent antenna for salt water locations. To achieve the energy at the low angles using Yagis is impractical. They simply cannot be placed high enough to compete with the SVDA and the Yagis have nulls at useful take-off angles.
Why use only 2 elements? Good question.
The answer is basic: the 2 element, parasitic array gives the most gain for the size and weight over a single element. In either vertical or Yagi implementation, the addition of the second element increases the forward gain in the 4-4.5dB range. The spacing on 20 meters is small, as close as 8', making a 2 element array compact and powerful. A third element requires the effective boom length (total spacing between elements) to be doubled for the next 1dB improvement. For more information on this, please look at Debugging an Antenna, "Part 5 On Air Observations", found on this same web site.
All the K5K SVDA's were fully tested before shipment. The last thing anyone needs is to be on a DXpedition and find some kind of problem. Each SVDA was carefully measured and a documentation package was prepared for each SVDA by band. The team was planning to use 1/4 wave verticals on the low bands, so an additional chart was prepared on how to elevate the radials for maximum efficiency (note: the simplest and most efficient radial system used as the current return for verticals over/near salt water is 2 elevated radials). Using elevated radials was again proven to be the right choice for the Kingman Team. The chart follows:
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This chart will aid in:
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The SVDA antenna can also be used over ground with good results. The main lobe will not have such a low angle component, but for limited space and no radials, it might be an excellent choice for locations besides those by the ocean.
K5K Team Testimonials and Facts:
* 80,841 QSO's is #3 among all DXpeditions to date, regardless of personnel, time on site and equipment..
* Highest QSO total ever for any pure boat, tent and generator DXpedition.
* Verticals easily surpassed tribander at 30' effective height above ground (overlooking the ocean, of course).
* Team says QSO total would have been 1/3 less if they had used Yagis.
If you have any questions on the SVDA, or any other Force 12, Inc. products, please contact us.