Fortunately, man has considerable experience in high-power microwave generation, transmission, and rectification. As early as 1963, Brown (18) demonstrated that large amounts of power could be transmitted by microwaves. The efficiency of microwave power transmission will be high when the transmitting antenna in the SSPS and the receiving antenna on Earth are large. The dimensions of the transmitting antenna and the receiving antenna on Earth are governed by the distance between them and the choice of wavelength (19). The size of the transmitting antenna is also influenced by the inefficiency of the microwave generators due to the area required for passive radiators to reject waste heat to space and the structural considerations as determined by the arrangement of the individual microwave generators. The size and weight of the transmitting antenna will be reduced as the average microwave power flux density on the ground is reduced by increasing the size of the receiving antenna and as higher-frequency microwave transmission is used. The size of the receiving antenna will be influenced by the choice of the acceptable microwave power flux density, the illumination pattern across the antenna face, and the minimum microwave power flux density required for efficient microwave rectification. a. Microwave Attenuation (20) Ionospheric attenuation of microwaves is low (less than 0.1%) for wavelengths between 3 and 30 cm and for the microwave power flux densities occurring within the beam. Tropospheric attenuation is low for wavelengths near and above 10 cm, but attenuation will increase as wavelengths are reduced. Moderate rainfall attenuates microwaves approximately 10% at a wavelength of 3 cm and 3% at a wavelength of 10 cm at a nadir angle of 60 deg. The efficiency of transmission through the atmosphere in temperate latitudes, including some rain (2 mm/hr), is approximately 98% and decreases to 94% for moderate (33 mm/hr) rainfall, depending on location (see Figure 4). b. Microwave Transmission System Efficiency The efficiency of the microwave power transmission system is a product of the efficiency of dc-to-microwave power conversion, the efficiency with which the microwave power is transmitted to the receiving antenna by the microwave beam link, and the efficiency with which the microwave beam is controlled, pointed toward, and intercepted at the receiving antenna, and there rectified to de. This overall transmission efficiency can also be measured experimentally as a ratio of the de power output at the receiving antenna to the de power input at the transmitting antenna. Table 2 indicates the efficiencies that have been demonstrated in the three major functional categories of the microwave transmission system and the projected efficiencies which should be achievable with further development (21). Including microwave attenuation, the overall efficiency of microwave transmission from de in the SSPS to de on the ground is projected to be about 70%.
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