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Amateur Radio Emergency Service |
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Hilltopping on Two Meters
| (From an AE3C posting to the "Live-Wire" Yahoogroup) It sounds like you have an outing planned for "Hilltopping-on-Two" on the "2metres" band, (which is very close in frequencies to the US "Two-Meter" band). Much of the fun is the preparation. Here are two simple ideas and one more complex idea (sorry about using inches and feet to describe metric-ready aerials) for easy-to-carry Two-Meter luxury. #1 -- Bamboo Pole / Coat Hangar "OWA" Yagi First, try the Bamboo Pole OWA-Yagi, made entirely out of a six (or seven) foot bamboo pole and seven or eight straightened coat hangars. Notice the very, very close spacing between the driven element and the first director, which is what makes this an "OWA." When you use this antenna, you will rename it "WOW." Drill small holes, slowly to keep from splitting, into the bamboo. Just press-fit the straightened coat hangar elements into the holes, assembling the yagi at the hilltop. Be sure to use two separate wires for the driven element (they will be apart by a quarter inch or so), and feed with coax and alligator clips. (These dimensions are based on the OWA-W4RNL design, modified for wire.) Bamboo OWA-Yagi Element Lengths R 40.5" Dr 40.75" (split, of course) D1 40" D2 36.5" D3 36.5" D4 35" Bamboo OWA-Yagi Element Spacing (from the Reflector) R --- Dr 10.25" D1 14.25" D2 26" D3 37" D4 54.5" #2 -- Three-Dimensional Folded Loop This is a really clever idea from WX7G that I've tried using both copper plumbing (you have to flux and solder, though), and also bent stiff wire. The idea is to take a full-sized one-wavelength vertical quad-loop, and then take advantage of the fact that most of the radiation field is produced by the current near the feedpoint and also along the other vertical side of the loop. By keeping most of the vertical elements vertical, but then folding the rest of the loop into a box shape, you have a three-dimensional cube that radiates almost as well as the full-sized loop. Using stiff wire and two heavy duty pliers, it takes less than half an hour - start to finish - to make this antenna. A 3DFL at two meters is about one-foot cubed, and at 10-meters is 3 x 3 x 5 feet. AT 17 & 20 Meters, it is small car size. I found the construction details on the antennex.com page. Once built, there are endless ways of decorating the 3dFL antenna, as a pack frame, container, mail-sorter (my use for the Two-Meter 3dFL), glass tabletop holder, box kite, or armoire. #3 -- Two-Meter Full-Dimension Bobtail Curtain A full-sized bobtail curtain is really easy at two meters. A bobtail curtain (a popular antenna for 40 meters) is a co-phased array of three quarter-wave vertical wire elements that are spaced about one-half wavelength apart. The three elements are connected by horizontal wire elements at the tops of the verticals. The array looks like < ITI >, connected at the tops. The center element is fed (usually at the bottom against ground radials, but I have a twist here for two meters), and the horizontal wires co-phase feed the outer two elements. The bobtail curtain has a very low 'Figure 8' radiation angle, making it better for ground wave than for dx. Impedance at the feedpoint (the top of the center element) is around 30-35 ohms. Now here's the twist (from SM5DXV's idea): By replacing the center 'element' with a one-half wave feedline (possible at two meters due to length, but impractical at 40 meters), the bobtail curtain will now really be two upside down 'U's' looking kind of like this: < UU > (but upside down, of course) (I guess this is where the tail was bobbed.) The feedpoint impedance will be reflected at the end of the one-half wave feedline. Be sure to remember the velocity factor calculation for the center feedline length. When completed, this Two-Meter bobtail curtain looks like something that Alexander the Great would carry, so try attaching a flag or double-headed eagle of Lagash banner to it. Although all three of these aerials have really complicated theories, actually making, transporting, deploying, and using them is really easy. 73 de AE3C / Rick Pittsburgh www.pgh-net.com/ae3c |
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