27 August 2007

As of last Friday, we hadn't really picked an antenna yet, but have looked at several things, including a purchase of a W3FF Buddipole -- at a cost of almost $400.00. While it appears to be a very cool antenna system, extremely portable and used by several DXpeditions, we decided that was a bit out of our price range. Besides, having been a communications guy for a long, long time, and setting up almost anything in a pinch as an antenna, mostly wires, zip cord and military equipment, it was decided we build one and use as much stuff from home as possible and keep the price down. This of course, didn't preclude us from purchasing a few items at a hardware store this past weekend.

After a bit of brain storming over a couple of beers at our favorite food place, and quite a bit of drawing on napkins, we came up with a design, some calculations and then tried to come up with some kind of simple hardware we could make into an effective antenna.

We decided the following:

1) Multiband homebrew dipole.

2) It had to be light, shortened and we needed to be able to stand it in a free-standing mode with a tripod or some sort.

3) Wire dipoles were out because of the previous site surveys showing little or no antenna supports available (trees!) and we wanted something sturdy enough to stand on its own with little sag.

4) The antenna had to be fairly easy to assemble.

5) It had to be reusable, and it has to actually LOOK good as well.

We tried some extendable/expendable curtain rods and some fiberglass tent poles that are held together with an elastic string through the middle. Both sagged considerably, and were weaker than we expected. The curtain rod idea was good, but we weren’t going to use the rods as radiating elements, and couldn’t come up with a good way to put coils in place. We tried inserting wooden doweling into the curtain rods, then started thinking about the interaction of the metal with the actual antenna. That wasn't going to work well.

The fiberglass poles were just a bit heavier than we wanted, and again, with not a good way to mount things. They sagged as well, more than I thought was usable. Putting a coil and some wire along the fiberglass shafts would have worked fine, and simple electrical tape to hold it would have worked well, but the antenna would have been 16 or 18 feet long, even with the loading coils and it would have sagged miserably.

Then the idea struck us to use aluminum arrows. JoAnne had already mentioned arrows once, but as is typical for me, I was thinking one thing and she was thinking something else. I was looking at SUPPORT (and thinking wooden) and she was actually thinking of the technical aspects of using ARROWS (and thinking METAL -- Duh on me!).

So, on Sunday (yesterday), we purchased a handful of $3/ea shafts, removed the plastic arrow head inserts, and cut off the fletching and nock end of the shaft. This left us with a set of hollow aluminum tubes. Next we inserted metal inserts.

At this point, I only finished a few of the shafts yesterday afternoon, and we went off to a birthday party for one of our grandchildren. (Side note: We gave her some walkie-talkies, with a Morse code button.... so we're starting her EARLY, she's four and already knows her alphabet, so we figure by next year she will be getting her Technician license!)

Today, using a program found at K7MEM’s site, I calculated a slightly shortened, loaded dipole antenna. http://www.k7mem.150m.com/Electronic_Notebook/antennas/shortant.html

The actual “design” of the antenna will be similar to the W3FF Buddipole, including a tripod to hold the antenna up.

The center feed will consist of a T-section of PVC pipe, an SO-235 connector, loading coils for 20 meters and coils made from PVC pipe, end caps and associated hardware, screws and threaded arrow shafts. As of 27 August, the antenna is partially built and we expect to do some hard testing on 1 September 2007 to see how the antenna works.

My thinking is that we will use PVC piping as the mast section, cut into shorter pieces for transport in our suitcases on the trip. We'll use some PVC couplings to connect the pieces into a longer piece. The tripod we're going to use isn't the most sturdy thing in the world, but it was an old camera tripod and should hold this antenna up with no problem.

To build the coils I will be using short pieces of PVC pipe with end caps on each side, drill holes for the wire, and run the wires to hardware inside the pipe, allowing me to screw the newly made "Arrow Elements" to either end (on the end caps) of the coil for center loading on the antenna. I'll install an SO-239 connector on the T-section, to allow easy connection of the coaxial cable and probably install, if necessary some kind of ferrite beads at the connector to prevent RF from radiating from the coax.

Having tested some of the arrows by screwing them together already, I found that this is a very STURDY antenna element and shouldn't sag very much at all. Calculations are as follows:

Total length of dipole: 16 Feet long
Element Length Before coil: 4 feet each side
Secondary Element (Coil Plus element): 4 Feet each side

Coil specs:
Inductance: 8.76 uH
Diameter: 3 - 1/4"
Length: 1 - 5/8"
Turns: 10
Wire: #12

More to follow on this antenna. When I get it assembled, I'll try to post some pictures of it all set up so you can see what it looks like. We should be able to tune the antenna more carefully once it is put together, by adding or subtracting some length off the ends of the elements.

I think I'm going to call it a "DX-Stalker" because the arrows I'm using are made by Easton, and are called "Stalker" arrows. They are camouflaged. Perhaps we'll call it "DX-Prey" since, we'll be the DX station!

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