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#13
February 23rd 13, 11:53 PM
posted to rec.radio.amateur.antenna
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Homebrew 222 Mhz Beam Antenna Project
"Jeff Liebermann" wrote in message ... On Fri, 15 Feb 2013 23:31:52 -0800, "Sal" salmonella@food poisoning.org wrote: I'm building a yagi from the measurement tables the ARRL Antenna Book. Which Antenna Book and which yagi? I have the driven element finished and getting a good SWRs with a home-made gamma match. Think about using a T-match. Slightly better bandwidth and slightly more symmetrical pattern. However, I will conceded that the Gamma match (half a T-match) is simpler because it can be driven directly without a balun. I prefer a folded dipole driven element (200 ohms) and a 4:1 balun. With 200 ohms. I'm going to mount it on a wood 2 X 2 with as many directors as will fit on an 8 feet boom. The problem with wood is that it doesn't survive well on top of a tower. You could paint and preserve it, but by the time you're done, a metal pipe would probably have been easier. 2x2 also seems a bit light to support its own weight (plus any birds that may want to roost). I would suggest a 2x4. Wood is also hygroscopic (absorbs water), which might have an effect depending on how the antenna is built. Drivel: About 15 years ago, I had the semi-bright idea of building an all wood portable yagi for 440MHz. It was built as if it were a carpentry project complete with mostly non-metallic joinery. To turn it into a real antenna, I covered the wooden dowel elements with aluminum duct tape. http://www.homedepot.com/Paint-Adhesives-Tape/h_d1/N-5yc1vZ1xhmZarpz/R-100030120/h_d2/ProductDisplay?catalogId=10053#.USBOGfIrFR0 The T-match and balun were attached with U shape staples. Sorry, I didn't take any photos. After about month of tinkering, I turned it into firewood. The problem was that trimming this antenna was rather difficult. As the wood in the elements dried out, the velocity factor changed, causing the element tuning to change. Of course, I used the cheapest green lumber I could find, resulting in a continuously changing VSWR as the wood slowly dried out in the sun. You won't have that problem if you make only the boom out of wood. Any advice? (I've already read that a gamma match is a poor choice but it did work, so we can skip that one. :-) Measure the 2:1 VSWR bandwidth of the antenna. If it's too narrow to cover the 220 band, think about using a T-match and balun. With an 8 foot boom, you'll probably fit 9 elements: http://www.vk5dj.com/yagi.html 13dBi gain, 44 degree beamwidth. Idea: I was at the hardware store today and noticed some 2"x3"x10ft rectangular cross section vinyl rain gutter downspouts. A downspout might make a usable boom for a yagi. http://www.lowes.com/pd_12493-322-AW200_4294806370__?productId=1007997 It's quite light, doesn't sag, UV proof, and cheap. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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