would be an ellipse with a minor axis diameter of 34 km, and for 0.1 mW/cm2, it would be 16. 2 km. The difference in the area for the two examples is approximately 900 Ion2. Another important aspect of the rectenna is the conversion efficiency of the dipole element, which tends to fall off rapidly at low power densities. The baseline was constructed to keep the density at the edge of the rectenna at 2 mW/cm2. A design feature that has concerned designers of the rectenna is the waste heat of approximately 585 MW generated by a 90 percent efficiency of conversion. While this is a large amount of waste heat, it does not represent a problem to the rectenna. The 585 MW constitute an average of 40 mW per diode for each of the 13.6 billion diodes in the rectenna. This level of heat is well within the design limits of the diode. Future analyses will attempt to derive techniques for terrestial application of waste heat in the proximity of the rectenna. 8.2.3 COMMUNICATIONS, CONTROL, AND DATA MANAGEMENT SUBSYSTEMS The emphasis in this study has not been on the communications, control, and data management subsystems; therefore, detailed design requirements are not available. However, considering the functional requirements of subsection 8.1 and the central role of the SPS receiving site in the supervision of the system and operational activities, it becomes apparent that several communications and data management systems will be required. Typical communication links associated with space and ground interfaces have been indicated in Figure 8-1. The greatest most complex requirements will be those for monitoring and control of the microwave beam and the numerous elements of the SPS subsystems. These must be closely coordinated and functionally integrated with the control and data management subsystems servicing the rectenna and other subsystems of the receiving site. The command and adaptive phase control systems will be part of the central system with many sensors located in the safety zone. The size of the receiving site may dictate many intermediate data collection points, with RF relay to the central control system. If this is true, RF interference could become a problem. However, present technology should be adequate to solve such problems associated with the data management subsystem.
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