Radio Tower Design
Our second move, after one and half years, was to the home of a licensed amateur, who used the house as a holiday home. The house came complete with a fully equipped station and self supporting lattice tower, with a thunderbird tribander on top at 70 foot. The base of this tower was in the order of one and half meters square, tapering off at the top to about half a meter. The tower was climbable, but hard work due to the distance between tower runs not being very conveniently spaced. The tower was solid and of good construction, using local materials. It stood on a large single concrete foundation block. I believe that the tower was fabricated on the spot.
Our third move after a further period of two years, was to a location out of town, and into a bungalow. The bungalow sites in its own spacious grounds, approximate. 170 foot by 80 foot with the house towards the rear. Our condition for renting was that the owner provides a written letter to the telecom ministry, giving permission to erect a tower, and allow a radio station, which was agreed. See House and Garden Plan
We both loved the bungalow from first sight, with its style, and quiet rural location, fortunately it has turned out that we now have a long lease agreement with the owner. At last, I had the location that I wanted for the antenna farm, and my wife has her lovely kitchen. Thoughts now turned to constructing a tower. My license was still one and half years away at this point.
I was familiar with the STRUMECH range of towers, and wanted the experience of being able to wind the tower down and tilt the tower over for antenna building, and maintenance. Unfortunately, the import cost was prohibitively high. Import duty on items can be as much as 30% on C.I.F. I decided to have a go and provide some homemade drawings for local fabrication. I designed several models with using AutoCAD 2000 in 3D format. However I quickly realized that there are specific ratios of leg diameter sizes that telescope into the next section. See section drawings
As I extended the design of the tower to one hundred foot, the base section became alarmingly wide. Also the diameter of the leg tubes, necessary to provide the tensile strength, started to become industrial steel, and not just off the shelf galvanized water pipe. After considering all these factors, not to mention the fabrication difficulties, winch and cable pulley system, I rejected the idea of a home made crank up tower.
I visited an amateur who wished to sell his tilt over tower, and at this point realized all the mechanical implication of a home made tower and its safety factors. Especially when considering the required base foundations, the project was beyond my capabilities, on my own.
My next design escapade, was for a guyed fixed mast, similar to the Rohn 45 models, which seemed to fit the bill, but how to give the fabricator instruction how to make it without drawings. Again, I turn to AutoCad. This time in conjunction with GW Grape program, to analyzer the internal and external structural stresses. See details of program.
The program also takes into consideration, such as wind loading factors, and applies them to my model, displaying result graphically, and table formats, all sorts of stresses and allowable bending moments. I designed for a wind loading of 60 miles per hour on a 20 square foot head load. The highest wind speed I recorded at this location was about 15 miles per hour. I now started to get the feeling of diminishing returns, as again the materials were approaching industrial grade steel, and not readily available nor cheep in Thailand. This idea was also rejected.
About this time, my license arrived, (November 2003) and after four years, nothing was prepared, or decided, what I would use for a tower, or antenna. I must tell you that by standing near the main high way, not far away, it is impossible not to see a radio mast supporting some sort of antenna. I started to inquire who was making these towers and at what price. The answer was that each 3-meter section cost 20 pounds. Judging that several nearby masts, in access of 200 foot, had not collapsed, warranted further investigations.
I was lucky in contacting a member of RAST who knew of a tower manufacturer, in Bangkok, so off I went to Bangkok. The factory was excellent, with huge mast welding jigs, and all manner of galvanized hardware. They manufacture tower sections to order, either 12 or 15 inches wide, all triangular in shape. I ordered ten pieces of 3-meter sections, tower caps, shackles, equalizing guy wire plates, anchor bolts, the total bill including galvanizing, painting section red and white and a 500-mile delivery charge. All for under 500 pounds including a bottom section pier pin, and matching base plate.
Part of my initial design work, in auto cad, showed the location for the best fit, for the three anchor foundation blocks, once the mast location was finally decided. My mast design warranted a pier pin, from article that I read on the web, because in my case, perhaps all three guy wires may not be symmetrically 120 degrees apart. To my surprise the Bangkok manufactures knew of these problems, and offered a ready made pier pin base section, 1.6 meters overall length.
I was still unprepared, having cogitated about digging, four one cubic meter holes, when the lorry arrived with the tower pieces from Bangkok. I contacted a local friend who engaged a group of workers, all four holes were dug in a single afternoon. I am not at liberty to tell you where six cubic meters of earth went, but our defunct swimming pool is now not as deep as it originally was.
Another gang was hired, and reinforcing cages manufactured. Unfortunately three were oversized and well over one meter cubed. They did not fit into the hole very well. Three of the holes were dug wider with a margin for the concrete to flow around and under the cage. I estimated that each hole now require approximate one and half-cubic meter of concrete. This is well in access of the tower required minimum specification sheet, so an extra margin for safety was built in, because of the oversized re-enforcing cages.
Initially the contractor ordered two lorry loads of pre mixed concrete, but for some reason cancelled one load, not speaking or understanding the Thai language, I had little say in the matter. The work men finished hand mixing about three cubic meters themselves, to make up the short fall. Shuffling back and forth, for bags of cement and truckloads of sand and stones.
The Thai's are very innovated at solving problems, and soon lots of odd pieces of channel iron appeared to guide the wheelbarrows along, also to protect the grass. One enthusiastic work man tipped over his barrow of concrete, I had no idea how difficult it is to clear up the mess. Once trodden on, and set, cannot be located, until the grass leaves a bold patch, and then requires digging out.
The anchor cages were partly welded, and partly wired in traditional fashion, at times, due to the limited space between the re bar, was difficult to tip the concrete down the hole. On the good side, the concrete required packing around the rebar, every wheelbarrow load. So was pleased about that. I had shored up the sides and top with two by thee inch timber, so leveling out was easy.
Not being a civil engineer, I left 6 inches space, for topsoil covering. I read concerns of water ingress into concrete rusting the re bar, but argued that raising the concrete above grass level, was no assurance that prevented ingress any better. Water is still able to percolate down the side of the blocks. Time wise, the job took a full day, using five men, working none stop. Light was failing, just as they finished up, so start real early in the morning.
This I did using the same technique as lifting the antenna off the ground. The derrick and winch worked fine again. By anchoring the base of the tower to the pier pin, with steel rope to stop the tower slipping outwards, was able to winch all three section (12 meters) into an upright position. I then used a long 2 by 3 timber to lever the tower base, onto the base pier pin. The guy wire when installed cleared the roof gutter by 6 inches, as designed.
There were five people, one rigger, three helpers, including a young girl, and a man who turned out to be an interpreter for me. Within minutes, the girl was busy making up eye thimbles for the end of each rope, and fixing with bulldogs clamps. The three men each assigned themselves to an anchor point and remained solely responsible for all fittings. The rigger ascended the tower and sections zipped up like lighting. Ever one had a job, and they knew how to do it.
Raising the Antenna
The main obstacles are the tower support guy wires; the antenna must be pulled outwards to clear the upper most set, which the antenna becomes trapped in, due to the diminishing angle as the guy wires converge towards the tower. Hauling requires a minimum of seven people, three hauling on the up rope, one each stabilizing each rope, one up the tower, and one supervisor (me).
The secret is the gin pole on top of the tower. The gin pole must protrude higher than the rotator mast by several feet or more. Once the antenna has reached the top, the rigger cannot lift the antenna. It is the hauling crews job that must hoist the antenna past the boom mounting plate, so that it can be lowered and guided onto the fixing plate by the rigger on the top. He has enough to do hanging onto the tower.
In my case, it was at the riggers request that the antenna, rotator, stub mast, base-mounting plate, relays box, and coaxial cables tails, be hoisted as one item. Hence, three riggers on the up haul rope. The advantage in this method was that all the fixing, tightening, and cable harnessing is completed on the ground. Due to this unusual method, and confidence of the riggers, the job was finished and bolted up within half an hour, including fixing the LDF 450 coaxial cable to the mast, as the rigger descended.
I was pleased to see the old plum bob line, dangling from top, to bottom, of the tower. The riggers eyes replacing the usual theodolight for vertical alignment. In fact, it looked perfect to me from all angles, and suspects that initial judgment was made by the tension crew, on all three wires before the plum bob appeared. The tower installation was an enjoyable event. With only one person able to speak English, we all understood the job in hand. The final event that pleased me was that they wired up all the turnbuckles, true professionals, despite no hard hats, and wearing old rubber flip-flop shoes.