Subsequent studies (references 6-9) addressed the detailed requirements of the microwave power transmission system (MPTS). In particular, a Raytheon study (references 6-8) provided an assessment of critical technology areas, impact of uncertainties on system cost, preliminary assessment of environmental impact, as well as RFI impact. Also included was a suggested plan for development and verification of critical technologies and techniques. Another study (reference 9) addressed similar questions but also included a preliminary assessment of the economic impact of possible environmental and hardware constraints. In the course of these studies it became apparent that very large antennas would be required for power transfer from space-to-earth. The need for the receiver to fill the main lobe of the transmitter at a distance of 23,000-kw-led to the selection of an approximately 1 km diameter transmitter and 10 km diameter receiver at 2.45 GHz. In principle frequencies above 2.45 GHz would tend to reduce the required antenna sizes. However, other constraints such as transmitter structural thermal limitations and peak receiver RF power density restrictions place a minimum size limitation on the transmitter and receiver and as a consequence there is little frequency advantage above 2.5 GHz. In addition rain losses as well as subsystem inefficiencies are of sufficient significance above 2.5 GHz as to actually cause system cost ($/delivered kw) to rise above 2.5 GHz. As a result, 2.45 GHz would be an effective transmission frequency, being in the appropriate
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