Thursday, July 24, 2008

My first beam - Part 2

In case you missed it, click here to read Part 1.

Now that I had all the parts laid out, I re-re-re-re-read the instructions. First things first: I took out the tools that I needed (listed at the beginning of the instructions) which consisted of a couple of difference sized wrenches, one socket wrench, a screwdriver (oops, grabbed a flat blade instead of the Phillips head; I had a 50-50 chance and guessed wrong), and a tape measure. Amazingly, I actually had the right sized wrenches and socket. (Well, not so amazingly, they were common sizes.)

I wanted to be very careful before tightening any of the locknuts, since I didn't want to have to undo any mistakes, which might have affected the "holding power" of the locknuts. I knew I couldn't mess up the first step, which was connecting the two boom sections together. There really only was one way to do it, since the screw holes on both halves only lined up one way.

The next step was to mount the reflector element on the back of the beam. By way of explanation, the 6m3 antenna is a three element Yagi antenna, a very common type of directional antenna. The link gives a more in-depth explanation, but in a three element yagi there is an element in the middle which is actually connected to the transmitter called a driver, a slightly longer element located electrically "behind" the driver called a reflector, and a slightly smaller element in front called a director. It's important to get these in these set up in the right order or else the antenna just won't work right.

The instructions say "Mount the longest element ... to the hole at the rear end of the boom...". Ok, so, um, which is the "rear" end? The diagram does show spacing between the elements, but I took a quick look at the diagram, figured that the drawing showed the rear of the beam at the bottom of the page "pointing" upward. Of course, had I actually looked a little more carefully I would have noticed that the longest element was at the top of the page, not the bottom. So, I promptly proceeded to put the reflector element on the wrong end of the boom. Fortunately, I intentionally didn't tighten it up all the way, and when I figured out what I'd done wrong, I was able to move it pretty easily. By that point, I'd put on the driver element, which didn't need to be moved, so I only had to move the one element. No harm done.

The rest of the assembly went pretty smoothly. I put on the other components, and attached the balun to the T-match as instructed. In this case, the balun is in the form of a quarter-wavelength of coaxial cable that you're supposed to loop and attach to some connectors then secure it to the boom with the supplied black tie-wraps. (Black tie-wraps are UV-resistant,so they don't break down in sunlight as quickly as regular tie-wraps.) The only problem is that when I did the initial unpacking on the antenna, I must have unpacked the tie-wraps (I'm pretty sure I saw them) and put them down somewhere. Unfortunately, that "somewhere" must be the same place as where missing socks go when put in the laundry; that is, in "never-to-be-seen-again" land. Amazingly enough, I actually had the exactly type of tie-wraps called for in my toolbox, so I used my own. Another bullet dodged. (Ok, another trip to the hardware store avoided.)

Now that I'd gotten all the parts attached and carefully checked the that the shorting bars (used to tune the antenna among other things) were where they are supposed to be, I re-read the instructions to make sure that I hadn't missed anything. The only thing remaining was to attach the boom plate to the boom The boom plate is used to attach the antenna to the mast. There are a couple of U-bolts plus nuts and lockwashers that you first attach to the boom, along with what's basically an adapter to allow you to connect the flat mast plate to the round boom. I will admit that it was a little confusing. There are a lot of holes on the plate, and it's almost like a puzzle: The smaller U-Bolts, used for attaching the plate to the antenna boom, and the larger U-bolts, used for attaching the mast plate to the mast, will fit into the plate in several different ways, but there's only one way that everything will fit in correctly. I have to admit that I probably tried pretty much every possible combination (unsuccessfully, of course) before figuring out the magic combination. One assembled, the solution is obvious, needless to say.

To be continued...

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