Building the Concrete Form for the Base Mounted Rotator ~ Updated 2X
Today we will cover the construction of the temporary wooden structure that will provide the form for the concrete base. Our initial calculation was that the reinforced concrete base would be about one foot cube with a portion above ground and the remainder below grade. While that seems sufficient there are other critical dimensions which include the actual area of the top surface.
The US Tower base mounting plate is essentially triangular since it is intended to be mounted within a triangular steel tower. My selection of the RP-3 (the smallest) is identified as "fitting" within a 10 inch top section thus we have approximate dimensions, likely the triangle altitude (known a "h") being close to 10 inches --well it is something less than that. The baseplate assembly, L brackets and anchor bolts are shown below, and were described in 9a.
At times it is the "practical" that drives the final solution/configuration. I had purchased two pieces of 1X4 pine each four feet long with intent of using these for constructing the concrete form. This is where I go nuts -- a 1X4 is not actually 1" X 4" but something less than those dimensions --but the four foot long was good. Thus if I made each side of the form 16 inches long that would be a total of 64 inches. Three saw cuts of one piece of the 1X4 would yield three sides each 16 inches long and a 16 inch cut of the second piece would give the fourth side. I offset two of the sides by approximately 1 inch (actually to avoid a knot on one of the pieces) and given that the 1X4 is less than one inch along one of the dimensions my picture frame box has internal dimensions more akin to a rectangle than a square, which is OK.
The final size internal size of the frame is 13 X16 X 3.5 inches. This equates to about 0.42 Cubic Feet of concrete which says the hole will have about 0.6 Cubic Feet of concrete. If we looked at a hole of about 9 inches deep and 12 inches in diameter that is slightly less than 0.6 Cubic feet. Thus we now have our dimensions for the base
Some math here and we can calculate the cubic area of the rectangular area and then determine the depth of hole so that we end up with a foot cube of reinforced concrete.
The outfall of the second piece of the 1X4 by four feet long was cut to form two pieces which straddle the picture frame box. The rotor base plate will be bolted to these two pieces and the two pieces anchored to the picture frame during the placing of the concrete in the form and the hole in the ground. Bottom line ~ this process results in the rotor baseplate assembly being placed squarely in the center of the concrete base at the proper height. When the concrete is cured the baseplate is unbolted from the form and the anchor bolts are in the exact position they need to be. I am a bit ahead of my self here.
[As an aside by having enough of the threaded portion of the anchor bolt be above the top of the concrete, it would be possible to have two nut assemblies on each bolt. One nut would be on the bottom side of the L bracket and the other (with a washer) on the top side. Such an arrangement could enable precise leveling of the baseplate assembly as the nuts underneath could provide three points of precision adjustment so that the baseplate is absolutely flat.]
Stepping back I temporarily placed the two pieces of wood that will straddle the form on top of the form. I should note that one of the piece was cut so that it would exactly straddle the form being net with the outer dimension. Why? Well as it turns out when the rotor base plate is affixed to the form one of the mounting holes will be equidistant from each side. So by finding the center of that piece of wood a 1/2 hole drilled at that location will properly align with the hole in the L bracket affixed to the base plate. We now have accurately located one of the mounting holes smack in the center of the form.
The remaining two holes of the baseplate are "in line" with each other. Here having the length exactly the width is unimportant. Thus I simply found the center of the 4 inch dimension of the 1X4 and drawing a line approximately 10 inches log aligned that length with the center of the two remaining holes. A quick scribe of the inside diameter of those two holes on to the line on the board with a felt tipped pen gave me a drilling template for the second piece of support wood. Once the three holes are drilled into the wood, the assembly complete with rotor mounted can be placed on top of the wooden form. Since one of the boards aligns with each side and since that mounting hole is in the middle we need only slide the assembly along the form until there is an equal amount of space on either side of the rotator along the other dimension. Pre-drilling of the straddle boards enables once the "center" location is found to simply screw the rotor assembly to the form. Use a total of 8 screws, two at each end so the assembly is rigid.
When you reach this point you should have an assembly that looks much like what you see in the photos below. (Additional Photos added of the completed assembly -- the anchor bolts are upside down just for these photos so the form for the concrete lies flat on the driveway.)
Having the rotor on the baseplate is important for the next phase which will entail placing a portion of the mast in the rotator assembly. Thus the baseplate, rotator and partial mast will be taken to the outside location where the antenna will be located. Once at that location, the assembly will be moved around a bit to determine the exact final location. My design not only entails the use of the baseplate assembly; but also a house bracket sleeve bearing. We know the proximate location of the base location but now we want to conduct a fit check so that when the base is anchored in the ground the mast with sleeve bearing is a proper distance from the house and rotates feely within the sleeve bearing. We will now cover that part of the process.
73's
Pete N6QW