the waveguide test fixture in the "up" or "down" position. The "up" and "down" positions corresponded to the position of the aluminum nuts which were used in conjunction with teflon machine screws to hold the side rails of the rectenna element together. The test procedure was to make an efficiency measurement in the "up" position, take the element out of the test fixture and reinsert it in the "down" position, take another efficiency reading, and to repeat this enough times to obtain reliable statistical data. The efficiency measurements in either position showed only a small amount of dispersion but the difference in the average efficiency obtained for the two positions was 0. 64%. It was then found possible to reduce this difference to 0. 2% by substituting teflon nuts for the aluminum nuts. There was also a net gain of . 15% in efficiency in the higher efficiency position indicating that the aluminum nuts themselves were a minor source of efficiency loss, independent of their unbalancing impact. The reason for the impact of the orientation of the rectenna element in its test position upon operating efficiency is traceable back to an interaction between the rectenna element and the metalic cylinder which surrounds a portion of the rectenna element. Ideally, the two-wire transmission line should be electrically centered in the metallic cylinder. If it is not, a plus-plus mode contamination superimposed on the normal plus-minus mode of the transmission line will result. The harm that this does depends both upon the amount of the imbalance and what impedance the plus-plus mode couples into. If the rectenna transmission line itself is electrically symmetrical and if it is mechanically centered in the metallic cylinder, the amount of imbalance will be negligible. However, the old design was not electrically symmetrical in that aluminum nuts were used on one side of the transmission line while the heads of the teflon screws protrude on the other side. ' The 3-48 thread on the diode also protrudes from one side which is the same side that the metallic nuts are on. Now, when the rectenna element is inserted into the fixture, and if the center line of the rectenna element corresponds to the center line of the metallic cylinder, the side of the transmission line with the nuts will couple more closely to the cylinder and set up an imbalance which will produce an efficiency loss. Presumably if the rectenna element is taken out, rotated 180 , and replaced, the nuts will be on the other side of the cylinder, and the imbalance and efficiency impact will be the same. But if the axis of the cylinder and the rectenna element do not coincide, then the results of the 180 degree rotation will be different; there will be more electrical imbalance in one position than for the other and the result in efficiency measurements may be noticeable. * The use of aluminum nuts in the design of the RXCV rectenna element was based upon better delivery and lower cost of aluminum nuts and tests which were not refined enough to detect an appreciable difference in operating efficiency of the aluminum and teflon nut designs.
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