TOWER SUBSYSTEM

Decisions-Decisions

    Because I needed a tower that would some-day handle a full-size 3L 40M beam, I needed a "bigger-than-average" tower. After looking at the capacity limits of several towers and talking to local hams, I looked seriously at the ROHN 55 tower. I also had an opportunity to purchase a used HD AB-105 tower from KE3Q. So, I needed to make a decision.

    I looked at ROHN 55 as well as the HD AB-105. The HD AB-105 was a surplus military tower, 24 inch face-width, bolt-together (1/2 inch bolt/lock washer/nut) that was procured from a local ham who retired and moved to FL. But, there was only 120 feet of it! How much did I really need?

    Well, for a 134 foot tower I ran the following calculation: 134 + the base section (5.5 feet), plus a small (5 foot) test tower to use in my basement for testing the various mechanical mounting devices I would be attaching to the tower.

    The concept of a local test tower turned out to be a very good idea, since I would need to fabricate several mounting mechanisms over the course of the design/implementation phase of a new tower, and, in addition, I would have the ability to test these devices in my basement/garage without needing to go to the tower to try them out. This helped immensely, both during the design phase before the tower was installed as well as afterwards.

    So, I calculated that I needed at least 145 feet of tower to accomplish my goal. After determining that I could procure and re-galvanize the old HD AB-105 for less than the cost of new ROHN 55, I decided to go with the HD AB-105. I obtained the requisit tower from KE3Q and W3LPL. I dismantled the tower sections and brought them home. My XYL was amazed at the project I had decided to take on.

    I then brought the tower pieces to Southern Galvanizing in Baltimore. Some of the sections were painted and the acid bath at the galvanizing plant would not take off the paint. So, after having the tower transported to Laurel, MD and sand-blasted, it was finally regalvanized. Boy does freshly galvanized steel look *real good*. I began the preparation for the installation in the spring of 1994.

    Welding the Base

    After much discussion with the local hams who were tower experts, We decided to weld (rather than bolt/washer/nut) the tower parts (legs, horizontals and diagonals) that would be in concrete. We did this so that there would be no difference of potential between any of the steel tower parts in the concrete - for lightning considerations.

    The Tower Installation

    The hole for the tower (specified as 3' x 3'x 6') was dug by a local backhoe contractor. Once the hole was ready, I was ready to set the tower base. I had fabricated a 19'6" base section (designed for 5.5' in concrete, 14' out of the ground) so I would be able to guy that short section in place while the concrete was poured into the hole. Well, I nearly over-engineered the project! I had completed the fabrication of the tower base and it was on saw horses in my back yard, ready to be set into the hole. But, my contractor had already dug the trench for the conduits and that trench blocked the back-yard path for the completed base section of tower (that weighed nearly 300 pounds) to the hole. Fortunately, there was another path around the house. So, the backhoe, being still on site, was pressed into alternate service.

    But, the backhoe was not agile enough to set the tower in the hole - it needed human helpers and the backhoe operator and I were not enough! So, we set the tower on the ground awaiting some more human strength. I put a call out on the local 2M repeater and was able to get 8 guys to show up at my house in 1/2 hour. We picked up that tower section and placed it ever so gently into the hole and guyed it down!!

    The rest of the tower (120 feet) was installed like an erector set: piece-by-piece. That is, we used a gin pole to haul up each 10 foot leg, one at a time, bolt them in place, then install the horizontal and diagonal braces, climbing up as we went. When we reached the top of each 10 foot section, we moved the gin pole and repeated the process. By the way, the gin pole we used was one with a clamp for angle tower legs. Gin poles with a clamp for tubular tower legs will not work with an angle tower. The HD AB-105 tower weighs about 140 pounds per 10 foot section, far too heavy to gin up as a single assembly.

    As we installed each bolt/lockwasher/nut, we did not tighten them to full torque, but left them *interestingly* loose. As we reached each guy-point, we attached the guy wires and adjusted them to make the tower reasonably plumb. Once the tower was complete, we used the services of a local tower company to plumb the complete tower, adjust the guy wires, and then we finished tightning all of the nuts on the tower bolts.

    Plumbing The Tower

    We took care to ensure that the tower was plumb by installing a plumb-bob system approximately 10 feet along one of the vertical legs of the tower. First, we took two equal size aluminum blocks (4" x 3" x 1/4"), clamped them together and drilled a plumb-bob hole and two mounting holes through both pieces. Then, using existing mounting holes on the tower leg, we mounted the two plates on a tower leg, directly above/below each other with 10' vertical separation between them. We then attached the plumb-bob line and weight. That provided the ability to align this short 19.5' tower to plumb as we guyed it. The plumb-bob also allowed us to ensure the tower remained plumb as we poured the concrete.

    It took a while to get the mud out of the base hole! Note in this photo, the welded tower sections that would be in the concrete - marked with the electrical tape on the tower. This electrical tape mark marked the level at which the tower would contact the poured concrete.

    Rust Prevention

    After much discussion on the local 2M repeater, we decided to coat the tower legs with roofing asphault/tar over the distance where the leg would be 6 inches out of the concrete and 12 inches into the concrete. This was to minimize the possibility that moisture would collect at the tower leg-concrete junction and potentially rust the tower legs. Both Frank, W3LPL, and I had such rust occur on other towers and this was a preventative measure.

    Once the tower was guyed in-place, and had passed the Montgomery County Maryland foundation inspection, we ordered the concrete (3000 psi 28 day strength) and poured on September 10, 1994.

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