5.4.1 Considerations in the Design of the Outer Metallic Shield In this design, the metallic shield fulfills three requirements imposed upon it: (1) the prevention of direct radiation from rectifier circuit into space, (2) to function as a structural element for the rectenna, and (3) to be fabricated economically. A photograph of the shield as mechanically fabricated is shown in Figure 5-9. The top and bottom parts are identical. The form of the shield and its appearance when combined with the rectenna core are shown in Figure 5-10. The choice of the kind of metal and its thickness for the fabrication of the shield received considerable attention. Aluminum was selected as the initial material, primarily because it represents an excellent tradeoff between cost and durability. A secondary factor was its low electrical loss, which is of some importance because some currents will flow in the walls of the shield. Stainless steel might also be considered, but it would be considerably more expen- siven even after allowing the use of a thinner material because of the higher modulus of the material. The requirement for low material cost which will ultimately represent most of the cost of the rectenna indicated that the stock should be as thin as possible since material cost is approximately proportional to thickness. On the other hand, the strength of the shield as a structural element, the deflection of the shield due to an external force, and the period of vibration will be dependent upon material thickness and upon the size and shape of the cross-section. Further, if the external force is caused by wind pressure, this force will be a function of the size and shape of the structural element. Although this situation would seem to generate a complex tradeoff, matters are greatly simplified because the size of the tubular shield must be at least 1*0 cm for mechanical reasons of housing the rectenna element core and to allow for adequate insulation in the grommet that fits between the shield and the antenna, but no larger than 1.5 cm because of increased wind resistance and non-TEM modes that could exist for high-order harmonics in larger tubes. A good compromise would seen to be a 1.25 0 cm tube, and to make it rectangular rather than circular in shape because of the increase in moment of inertia by a factor of 5.33 and therefore greater stiffness and strength for a given wall thickness, and because of the much easier problem in designing a suitable grommet. The problem then narrows down to a selection of the thickness of the material from which the tube of square cross-section will be made. This, in turn, will depend upon an appreciation of how the metal shields fits into the overall rectenna as a structural element. From Figure 5-1 it is seen that the tube is joined onto a foot which would normally be a part of the same continuously formed piece and that this foot, in turn, is joined onto the reflecting plane made from some kind of an expanded metal or interwoven wire (with secured joints at the crossovers). Thus, the moment of inertia of the metal shield arounds its
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