neutral axis parallel to the metal screen and at right angles to the axis of the tubular shape will be much larger than the moment of inertia on the structural member about a neutral axis normal to both the screen and the axis of the tubular shape. It is noted that the screen itself will prevent the foot of the structural member (that is, the point at which it joins to the metal screen) from deflecting up or down, and the screen itself will add to the moment of inertia because it is joined to the metal shield. This will probably place the neutral axis not too far from the foot of the structural member itself. This all adds up to a very small amount of deflection in a direction normal to the reflecting screen. On the other hand, if Figure 5-1 is examined, the metal shield as a structural element is seen free to flex at the point where its foot joins the metal screening. This could normally be taken care of by inserting some webbing between the foot of the structural member and the square tubular section and welding it to all metal interfaces. The method of continuous fabrication of the shielding member, however, makes the insertion and welding of such a web a difficult undertaking. On the other hand, it would be possible to attach a supporting member to the post (the post is shown in illustration 5-1 as a "T" beam). The supporting member would consist of two parts : an upper part which would prevent any upward motion of the tubular portion of the metal shield and a lower part which would prevent any downward movement of the metal shield. Then, between any two vertical supporting posts (the ” T” sections again), the bending of the tubular portion of the shield halfway between the supporting posts would be constrained by the moment of inertia of the tubular section about an axis normal to its own axis and normal to the supporting screen, and by the constraint placed on the foot of the shield where it joins the reflecting screen. A consideration of all of these factors led to a choice of aluminum material in the range of 0.012 to 0.025 cm., Material in this thickness range is consistent with the material costs that have been estimated for the production rectenna. Rolls of aluminum shim stock in sizes 0.01 2. 0.020, and 0. 025 cm thickness were obtained. An initial selection of 0. 020 cm thickness was made. The 0. 0 20 cm thick material is much thinner than is normally used in the shop and outside the scope of normal shop familiarity and shop equipment. Temporary tools were used, therefore, in the laboratory to form the material as shown in Figure 5-9. Since the material was formed around commercially available ground steel stock, a high degree of precision in forming the pieces was possible. A simple drill jig was also necessary for positioning and drilling the holes for the grommets. The foot or turned up section where the shield would join the rectifying screen was not formed on the piece. For electrical reasons, we wanted to be able to vary the spacing of the dipole elements from the reflecting plane when it was tested in the central area of the 199 element rectenna.
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