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Friday, August 13, 2010

Building a Yagi-Uda antenna - Hack a Day

Building a Yagi-Uda antenna

posted Aug 13th 2010 1:00pm by Mike Szczys
filed under: wireless hacks

[Tommy Gober] built this Yagi-Uda antenna that has some handy design features. The boom is a piece of conduit with holes drilled in the appropriate places. The elements are aluminum arrow shafts; a good choice because they’re straight, relatively inexpensive, and they have #8-32 screw threads in one end. He used some threaded rod to connect both sides of the reflector and director elements. The driven elements are mounted offset so that a different machine screw for each can be connected to the appropriate conductor of the coaxial cable. The standing wave ratio comes in right where it should meaning he’ll have no trouble picking up those passing satellites as well as the International Space Station.

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Here's His How to...

DUX Homebrew Arrow Yagi
radio : by Grendel - August 7th 2010, 11:36PM

This summer I attended the TI-2 Space workshop put on by the ARRL and DARA in Dayton, OH. We spent 4 days learning how to make contacts with orbiting satellites like AO-27, AO-51 and the International Space Station, just to name a few. The antenna we used was the dual-band Arrow II Antenna. I've owned one for years and really like it. I wish more people had them, but I think most people think spending $140 for an antenna that can only handle 10W is a bit much.

My aim was to make a cheap alternative to the Arrow that is easy to break down for transport and storage. I really like the idea of using aluminum arrow shafts for elements; they are lightweight, straight, weather resistant, and fairly inexpensive. Another nice feature is the #8-32 threaded insert for broad heads that almost every arrow comes with.

I spent a couple of hours reviewing all the "cheap" and "ugly" yagi designs, as well as others like the "tape measure" and even a new-to-me "backpacker" design. They each had their own advantages and loyal followers.

I finally based my antenna design on one found in the ARRL Handbook from 1999. While not an exact replica, my design is very similar. I had decided to go with the through-boom design like the Arrow, as opposed to side-mounted because it is, in my opinion, cheaper. After buying 6 arrows and a quick trip to Lowe's I had a length of #8-32 all-thread and a piece of 3/4" conduit to use as the boom. I marked a straight line down the center of the boom to give me a point of reference, measured out the spacing holes, made sure I was drilling square and level and got to work. After the 3 holes were drilled, I cut 2.5" lengths of all thread for the director and reflector. Since I was going with the split driven element design of the IDX yagi, I would not need threading for the Driven Element, I'd use machine screws for that. Originally I planned to use nylon 8-32 all-thread to go through the boom for the driven element, and feed each side independently similar to the Tape Measure Yagi, but I couldn't find nylon all-thread, and the longest nylon screws to be found came up an inch too short.

I cut a length of RG-58 cable and soldered on two o-ring connectors to slip over the machine screws before screwing on an arrow.
Lastly I cut each arrow to the appropriate length, which worked out to:
- 19 5/8" for each Reflector arrow
- 18 1/8" for each Driven arrow
- 17 1/8" for each Director arrow
The measurements came from IDX's lengths, subtracting 3/4" (for the boom) and then divided by 2 to give me the necessary length for each arrow.
I used a cutoff wheel and my air compressor to cut the arrows to length instead of a hacksaw. The cutoff wheel leaves a clean cut as opposed to sawing.

After putting my MFJ-269 analyzer on the yagi, I found the lowest SWR to be about 1.4 right around the target frequency of 144.490 MHz. Not too shabby for a first yagi.
For those taking my lead, I found it helpful to feed the yagi as perpendicular to the boom as possible. It is also somewhat important for the O-ring feedline connectors to be on the same side of the boom as the arrow.

Here's a pic of me holding the 2m yagi. It's a bit larger than the commercially made Arrow.

tags: yagi radio antenna ham_radio

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