AB7E Tower Project
Page 5
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The HD-70 is a tapered freestanding tower, and the base gets anchored in concrete before the rest of the tower is added.  That puts some burden on getting the base legs at the proper angle, and an error of as little as one degree would put part of the weight of the tower outside the footprint of the base.  I found this little digital angle gauge to be very helpful to make sure each base leg had the same angle within 0.1 degree (the rated resolution of the gauge).  The gauge is normally used for setting blade angles on a table saw and very conveniently has a magnetic base.

In addition to the radial ground radials that will go in later (some heavy gauge wires with ground rods for lightning protection and zillions of smaller gauge wires for when I shunt feed the tower on 160m), I wanted to make use of the rebar cage as a ufer ground.  I found some inexpensive Cadweld molds for that purpose on eBay, as well as the corresponding copper thermite welding material in various measures.  The mold I was using required 65 gram portions of weld metal, which I obtained by dividing some 200 gram canisters into thirds.  That worked fine except that each cannister has a limited amount of starting material at the bottom, so I used standard 4th of July sparklers as starting fuses for the weld metal and lit the sparklers with the propane torch.  A propane torch by itself will not set off the weld metal --- the weld metal needs an oxidizing atmosphere to fire off and incompletely burned propane actually gives a reducing atmosphere around the flame.  When I remembered to do everything right the welds came out fine.

Notice the wire-to-flat-steel Cadweld connections on the tower legs.  It seemed to me that the Cadweld connections might be a bit brittle, so I used half of a standard ground rod clamp to mechanically anchor the wire to the leg and then made the Cadweld exothermic weld for a more reliable electrical connection.

Here is the rebar cage complete except for the top grid.  You may imagine how much fun it was to crawl around inside of that tangle making the various wire-tie connections, but it obviously wasn't impossible.

This is the complete rebar cage, with the addition of three 2.5 inch conduits for the various cables and control wires.  I only needed a form for the concrete on one side of the foundation because of the steep slope of the land, and I added some vertical rebar around the perimeter to build a CMU retaining wall after the footing is hard enough to work on.

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