Sunday, July 6, 2014

Flashback: The Doomed Beam Antenna

A few months into my Novice amateur radio career, I was beginning to feel the nibblings of the DX bug pretty bad. A magazine article and a project in the ARRL Antenna Book had me anxious to try building a small beam antenna. The antenna had a small boom and a moderate amount of gain and appeared to be something we could build at home. It was thus that at age 13, my first beam project lived, and died.

The antenna was a two element phased affair known as the “ ZL Special”. I am not sure where the name came from but it brought forth visions of working many of them on fifteen meters. There are literally dozens of articles about the design on the internet now, but then all I had was a few paragraphs in the ARRL Antenna Handbook ( 1961 Edition!)

The antenna is basically two folded dipoles spaced a short distance apart and connected with a transposed phasing line. The articles about it indicated that this resulted in a 135-degree phase difference between the elements. It can be built like a small two element yagi, and fed directly with 52 or 75 ohm line.

I had planned to make it with two pieces of TV twin lead and just hang it in the air, then wondered aloud if it could be made with a wood frame to support the elements. My dad once again jumped into the project and we soon had a frame built out of material from his carpenter's equivalent to a parts “ junk box”. He had material stored that was left over from numerous past projects and remodel jobs. It was to be a lesson in antenna construction for both of us.

It was decided the antenna would be mounted on one of the wood masts for my longwire. The longwire I had was suspended between two wooden masts consisting of 2X4's using rebar as steep angle guys.( My dad never built anything half-way. Those masts had triangular concrete foundations) It would be supported by a short piece of water pipe that would be attached to the mast with a pair of angle iron brackets that would be placed around the mast rather than weaken the wood mast by drilling holes through it. The angle iron came from an old bed frame that turned out to be a source of hardware for a number of antenna projects over the years!

The boom for the antenna was only about five feet long and was to be a 2X4 with a threaded floor flange being the means of attaching it to the water pipe support. The elements would be the twin lead supported by thin rippings of redwood. The twin lead was tied to the supports with monofilament fishing line at about six inch intervals. It was fed with 75 ohm twin lead. This and all my antenna projects were low budget, all built with what was available. The 75 ohm twin lead was two cents per foot in 1963.

The antenna masts were both already designed to be tilt-over affairs so attaching the antenna assembly was not too difficult. It took us the better part of a Saturday to put the beast together and get it into the back yard. At that point we learned just what “heavy duty construction” means.

Dark was falling when we were finally ready to raise the antenna. In order to lift all the weight of the wood mast, we got on the roof to pull on a rope attached to the top of the mast. W had the mast sitting it up on a sawhorse to have it off the ground a bit and to give us a little better pulling angle. Once we got it vertical, it was a matter of putting in a locking pin through the lower part of the mast and reattaching the guy lines. The masts were guyed with pieces of iron reinforcement rod such as is used in concrete and came down within two feet of the base of the mast, making the mast almost like a small tower. My dad really did not like the idea of wire guy lines cluttering up the yard.

There was no rotator for the antenna. We had a light rope dropped to allow the antenna to be pulled in different directions then tied off. It was about 22-feet off the ground. The brackets holding the mast were larger than the mast pipe with a sleeve pipe over it, allowing the antenna mast freedom of motion. The lower of the two brackets supported its weight on an old wheel bearing. My dad was pretty resourceful about such things.

There was little wind when we got the antenna in place. It was also about dark. It would be the next day before the trouble showed up.

Because it was after dark, the 15-meter band was dead and devoid of signals with which to test the antenna. It tuned up fine, actually loading up much easier than the longwires did on 15 meters. It also showed much less power line noise than did the wires. That might have been because it was directional and was turned at right angles to the power lines.

I called a friend of mine across town to get on the air to try it out. I started out with it turned toward him. He was across Waco from me, probably about 12 miles away by air. The initial word was good. I was much stronger there than I had been on my old inverted V and somewhat stronger than on the longwires. Turning the antenna 180 degrees showed a good drop in signal and turning it 90 degrees to him had me almost disappearing. The initial feeling was that it was working well.

The big test would be the next day. I had a weekly schedule with a station in the state of Washington and that would tell the tale of whether the antenna would really work better than what I had been using. Images of large numbers of DX QSL cards began to appear in my head.

The next morning, the wind was up a bit. As we backed out of the driveway to go to church, my dad and I noticed that the elements were hopping a bit in the wind. He noted that they appeared to catch the wind quite a bit.

By the time we got back from church, the wind had picked up quite a bit. I went out back to try to turn the antenna to the northwest to set up for my schedule. It was a real fight getting it swung around. Winds were up around 20 to 30 miles per hour by that time and it was obvious that something about the wood elements was really catching the wind. The ends were moving a foot or more up and down continuously. The wood mast was creaking and swinging within the constrains of its guys.

About that time, my friend had driven over to see what we had built and as he and my dad came out it was obvious that the antenna or mast might not survive. Rather than loose the whole thing, it was decided to try to take it down.

That decision was almost a disaster in itself. It took all three of us hanging on ropes to lower the mast that by that time was bucking and twisting wildly. We got the thing down, took the brackets off the mast and raised it back up where it seemed to heave a sigh of relief at just holding up the wires again.

The doomed ZL Special never came up off the ground again. Within days it was disassembled and the twin lead went into my own parts junk box for use another day. Thus were lessons learned at an early age about light weight antenna construction and the effects of wind loading!

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