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.
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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