HOW TO BUILD ANTENNA TRUSS
(Page 2 of 3)
Once the E.M.T. is placed in the frame, you'll have to decide how wide the truss you're going to build will be. Naturally, there are a number of different considerations involved in this decision . . . including strength and convenience. (If you have any doubt about the ability of the truss to handle the load you're going to put on it, then either calculate the stress, strain, etc. according to engineering procedure, or ask someone else to do it for you . . . using the information in this article.)
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The distance between the tubes will determine how the 1/4"-steel diagonals will be bent. Since the corners that you'll form in this basic (also called "Warren") truss will be 90°, you can figure out the length of each straight section according to the following simplification of the sine law: The diagonal length will be equal to the distance apart times 1.4. Thus the 1/4" rod used in 6"-wide truss will be bent about every 8-1/2 inches.
BENDING THE ROD
We use the brake shown in the accompanying illustration to form the 90° angles in the rod, and if you're going to make very much truss, we recommend that you build one like it. The tool will make your work go more quickly, and its length stop (which consists of a threaded rod jutting from the side of the device, plus a piece of 1" angle) will guarantee that the turns are the proper distance apart. With one person working the handle and another moving the rod (a task that involves simply flipping it and pulling it through a little), a 10' length can be formed in less than a minute. (Of course, to get a 10'-long piece of finished diagonal, you'll need at least 14' of straight material.)
WELD OR BRAZE
Take your section of bent rod and lay it between the tubes in the form. If your bends are perfect, the diagonals may fall right into place between the tubes . . . but we often have to use some C-clamps or locking pliers to force the rod into position. Either oxyacetylene brazing or gas or arc welding will produce acceptable joints between the tubes and the rod. Remember, though, that the tube walls are thin, and too much heat will burn them right through. (WARNING: E.M.T. has a zinc coating and should be welded only in a very well-ventilated area!) Also, be sure to work down the length of the truss, moving from side to side to prevent the heat from deforming the rod.
