Sunday, July 27, 2008

My first beam - Part 3

If you haven't already seen them, you might want to read Part 1 and Part 2 of this series first.

Now that I'd gotten the beam, including the puzzling-to-connect-the-right-way mast plate assembled, it was time to take it outside to check the SWR and to make a couple of test contacts. As I mentioned in Part 1, I assembled the antenna indoors because it was extremely hot an humid out. There was plenty of room on the flood of our "great room", and the sliding door that went outside seemed like an idea way to get the beam outside once finished. Unfortunately, I failed to measure the height of the door before making the assumption about using it to take the beam outside. It turns out that the door opening is 6' 7" high (2.01m) while the beam (for those of you who weren't paying attention) is 6' 9" (2.06m) long. Oops. I remember that when the room was being built, with the door in place, they had to remove both sides of the sliding door in order to get the big-screen TV inside. While comtemplating doing that, or thinking about removing the elements temporarily, Sharon suggested that if I twisted the beam through the door that it might fit. Fortunately, she was right, and the screwdrivers and wrenches remained safely in the toolbox.

Now that the beam was outside, I wanted to put it up in the air and check the SWR with my antenna analyzer. I have three five foot (1.54m) sections of TV mast from Radio Shack that I've used over the years, plus a stake that is made to fit into the bottom mast section. The idea is that you pound the stake into the ground, put a plate (that came with the stake) over the stake, then the mast onto the stake, resting on the plate. It's definitely not for permanent use, but for a lightweight antenna for temporary use when there's no wind, it works fine. If I needed to leave it up a little longer, I have a collar that fits onto the mast with holes for ropes to uses as guys, but for what I was going to do, the stake was just fine. Unfortunately, I've now used the stake enough to that the top part (which you hit with a small sledge) has flared out, making it impossible to fit the plate over (which isn't a big deal), but also making it more difficult for the mast to slide on. (By the way, the stake that I got was the last one in a closeout bin at Radio Shack a couple of years ago. If anyone knows of another source for these, please let me know, I'd like to pick up another one or two.)

I did manage to get the stake into the ground and put the bottom section on the mast on it. I then started to attach the beam to the mast, figuring that I'd walk up the mast and lift it on top of the section already in the ground. The beam itself only weighs 6 pounds (2.7kg), and even though the mast sections probably weigh more than that, I figured that I could lift it myself. That wasn't the problem, but what I'd forgotten was that with the mast attached, I still had to attach and temporarily secure the feedline, but could no longer lay the whole combination on the ground. I used a trick I learned at Field Day which is to rest the mast on a ladder, which keeps the antenna off the ground and allows you to work on attaching the feedline. Fortunately, since the beam was small and the mast was short, I was able to use a pretty small stepladder that we had in the garage, attached the feedling to the feedpoint and secured the cable to the boom and mast with a few tie-wraps.

Walking the mast and antenna up to vertical position wasn't difficult to do by myself, though it certainly would have been easier had I had a helper. (I certainly will have help when I get this up onto the roof.) Of course, what I failed to realize was that the 15 foot height put the beam right into some low-hanging branches from a tree, so I took everything down, moved a bit away from the tree, and repeated the exercise. Now the beam was free to rotate and clear of the leaves. Before connecting the radio, I used my antenna analyzer to check the SWR and impedance. While I'd hoped that I got everything put together well, I expected to have to adjust the shorting bars or some other component. I was very pleasantly surprised to discover that the SWR at 50.1Mhz was 1.0:1, and the impedance was 51 ohms. (By way of explanation, "perfect" would be 1.0:1 and 50 ohms; I suspect that the error inherent in the analyzer is more than one ohm, so this was, for all practical purposes a perfect match.)

The next trick was to connect the feedline to my radio which involved putting a couple of sections of coax together and feeding them through the back door and into my shack. This worked just fine, but wasn't exactly ideal considering the outside temperature. Since the 756 Pro II has two different antenna inputs, it enabled me to perform a comparison with the 6m loop on the roof. For reference, that loop is at about 30 or 35 feet (around 10m) so it's at least twice as high as the beam on the ground. Although the 6m contest was still on, there wasn't a lot of activity, but using the loop, I did hear a few stations from New England, so I aimed the beam roughly in that direction. Sure enough, the stations were noticeably stronger as compared to the loop. I also noticed that many of the "birdies" (constant signals in fixed locations) were gone. I later realized that this was because at the time the beam was pointing roughly north-northeast, which was directly away from the house. When I pointed in the other direction, the birdies came back, which means that the cause of the birdies is most likely something inside my house. (On my "one of these days" list is to run the radio from a battery and to start shutting off circuit breakers in the house until the birdies go away, which should help me to figure out the source of the problem.)

Since it was getting late, I'd only made a couple of quick contacts, so I took the beam down and found a place in the garage to place it. I still need to order a rotor, then K2NUD has offered to stop by and help get the beam up on the rooftop mast. For now, I'll normally use the loop, but if I have time, I'll try to set up the beam temporarily.

2 comments:

  1. Thinks so much for putting this series up -- as a new ham, I've greatly enjoyed learning about antenna construction and installation.

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  2. drmello,
    I'm glad that you enjoyed the series. Thanks for writing.

    73,
    David, K2DBK

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