Base Construction
Wednesday, 06 July 2011 13:52 | 
Written by Peter | 
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I purchased the 6m lengths stainless steel RHS material from Alfa Laval. It was delivered to my home. The problem with using stainless steel for the main structure is that all connections have to be made with fasteners. Welding stainless steel requires special equipment and qualifications. I did not want to involve a metal shop so I decided to go with the fastener method. The thing I didn't see coming was that drilling the holes for the fasteners also requires a certain knowledge. I killed several drill bits before I went to the computer and did some research.
Stainless steel must be drilled at low speeds applying a lot of pressure and the application of plenty of cutting oil. Frequent stopping the work helps keeping the temperature of the steel and drill down. If stainless steel gets too hot it hardens and the hole cannot be finished. That can become a real problem when there is no space to drill another one.
I cut up one length with an angle grinder to make the two posts forming the base of the mast. I had read that 10% of the total length of an antenna mast should be buried into the ground to give sufficient support even in high winds. Having planned to go about 10m up I figured that about 90cm "in ground" would be enough. I used a probe to gauge how deep I would be able to dig because our property is pretty rocky. As it happened that was how deep I could go at the chosen spot before I hit sandstone.
The base is about 220cm long with the pivot bolt sitting in the middle. That leaves enough room for the cables to come out of the centre pole without being to close to the ground. All fasteners are made from stainless steel too. The only parts being galvanised steel are the L-piece at the bottom of the base and the brackets to secure it to the base. I added the L-piece thinking that it would act like an anchor to minimise the effect of the forces when the mast is being lowered or pulled up. I didn't do any research on how the longevity of the connections between the stainless steel and the galvanised steel parts is. Electrochemical activity can eat away one of the materials when the chosen combination is a bad one. Time will tell. Do you know if it is a bad one?
The hole I dug out for the base was intended to be filled with concrete. Therefore it is wider than it would have to be for a gravel filled base. I haven't noticed any adverse effects yet apart from lugging about twelve 25kg bags with Blue Metal up the hill.
After lowering the base into the hole I secured it with the help of a folding stepladder placed over the hole and the base strapped to it. Then I began filling the hole with Blue Metal. After each bag I compacted the gravel with a two meter long crowbar jabbing it repeatedly into the gravel. Quite exhausting work. I also checked frequently with a spirit level that the base was still upright. After every second bag I wriggled the base to and fro and sideways to press the gravel into the sides of the hole followed by more compacting work with the crowbar and an upright-check. When the hole was filled up I left the base resting for a couple of days.