in a string-like construction in which the diodes themselves were part of the collection process as well as the rectification process was developed and used successfully in the early helicopter work. (Figures 1-2 and 1-4. ) In time coincidence with the demonstration of the helicopter, Hewlett Packard Associates had developed a new physical format for a silicon Schottky barrier diode with the potential for much greater reliability and power handling capability than the point contact diode as well as offering considerably greater efficiency. A number of these were forwarded by HPA for evaluation and their superiority as rectifiers was confirmed by R.H. George (18), They were not put into a rectenna element format. In the time period from 1965 until 1970 there was no direct support of rectenna development from either government or industry. During this time period, the Air Force did support the development of a helicopter which would automatically position itself over the center of a microwave beam, a capability necessary for the practical use of a microwave powered helicopter. However, a substantial amount of development work on the rectenna was carried out by W. C. Brown using private funds and time during thel967 to 1968 time period. This, work was primarily aimed at designing a very light weight rectenna structure-which utilized a rectenna element format consisting of a half-wave dipole antenna terminated in a full-bridge rectifier made up of HPA 2900 diodes ' ’ " (Figure 1-5.) This work was important in that it established the physical format of the rectenna development effort that was to be undertaken later at MSFC and that was also to be supported under MSFC contract at Raytheon Company. It was also used in a demonstration of microwave power transmission to the MSFC Director, Werner von Braun, and his staff. This demonstration may have been a decisive factor in a decision to undertake the support of this work at MSFC during a time period of NASA contraction. In Spetember 1970 a demonstration involving the measurement of the various efficiencies in complete microwave power transmission system (DC to DC) was made at Marshall Space Flight Center ( 8) (Figure 1-6). The rectenna used for this purpose. Figure 1-7, employed rectenna elements patterned after those just discussed but developed to the point where their individual capture and rectification efficiencies approximated 70%. The configuration is important in the context of rectenna development for satellite power stations in that the collection, rectification, and DC collection was performed in a single plane positioned approximately a quarter wave-length above the reflection plane. This is the intended approach whose development was a part of the activity under this contract. The MSFC demonstration of 1970 indicated a number of deficiencies in the system including a rectenna collection efficiency of only 74% versus the theoretical maximum of 100%. This low collection efficiency was associated with improper spacing of the rectenna elements from each other in the rectenna array. It was therefore decided to space the elements more closely to each other and, in addition, terminate the DC output of each rectenna element in a
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