Because the shield was used for an experimental assembly containing the rectenna core which was modified during the testing period, the edges of the shield have holes drilled in them to permit the use of 0-80 machine screws to hold the pieces together. When the two halves are joined together with machine screws, a surprisingly stiff member results. 5.4.2 Considerations in the Design of the Core Assembly It was determined early in the work performed under the contract that the individual rectenna element would work satisfactorily in the waveguide test fixture if the axis of the rectenna element was oriented as shown in Figi. 3-4. Most of the effort in the design of the 5-element foreplane assembly was in connecting the five elements together. This normally simple problem was complicated by the fact that the pitch of the elements (that is the distance between centers of the successive halfwave dipoles) was not great enough to include the shorted quarterwavelength section of air line that is necessary to isolate the elements from each other at the fundamental frequency of 2.45 GHz. This problem was resolved by adding a capacitance in shunt with the input of the shorted section of line to resonate it at the fundamental frequency. From a physical point of view, this was accomplished by using a thinner Teflon washer at the input of the low pass filter so that there is no additional cost or mechanical complication. The solution is much superior to filling the section of line with a continuous dielectric. By adding another capacitance of the proper value at the midpoint of the section of line, it was possible to make the section an electrical halfwavelength at the second harmonic while still maintaining a quarter-wavelength at the fundamental with now another value of capacitance having been selected for the input to the line. The half-wavelength line at the second harmonic can behave as a short for the second harmonic at the output terminal, thus in principle decreasing the second harmonic output. However, the adjustment of the value of the midpoint capacitance was found to be very critical; and it was found that a decrease in efficiency and an increase in harmonic content occurred at a point of adjustment only slightly removed from the point at which the radiated second harmonic power was a minimum. It was felt that the reduction in efficiency might correlate with an increase in the generation of second harmonic power in the rectification circuit and suggested an investigation of harmonic power generation as a function of the termination of the low pass filter at the input to the dipole antenna. Although such an investigation was needed, it was identified as a diversion to the main thrust of this task, which was to test the foreplane as part of the 199 element MSFC rectenna. The midpoint capacitance, therefore, was not included in the construction of the core assembly. Before concluding this discussion it is noted in Section 3.4 that the second harmonic can be reduced considerably by tuning the intervening section of line to the second harmonic. The criticality of the adjustment was not noted although it may well have been there.
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