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