The microwave beams shown in Fig. 2 would not be visible to the eye. The transmission regions on the moon would likely be 30 to 100 km across and just barely visible from Earth. When orbital mirrors are eventually used to illuminate a base in the lunar night, one would see from Earth a bright diamond just inside the dark edge of the moon. Multiple illuminated bases would appear as a string of pearls. The bases could be clearly discerned from Earth during construction and operation through a small telescope. People all over the world could see the progress. A single segmented antenna farm 100 km across could supply over 10,000 GW to Earth through the multiple beams. This corresponds in power to burning about 1 billion barrels of oil a day. The world now consumes 0.08 billion barrels a day. For all practical purposes, the power beams from the moon could be as closely defined as shown in Fig. 2. Virtually all the power, except minor amounts scattered by the atmosphere and rectenna, would be inside the beam. Such well defined beams are allowed because large segmented antennas, like billboards, can be built on the moon of foamed glass, glass fibers, and iron or aluminum extracted from local materials. The segmented antennas would be positioned in an elliptical area near the lunar limb so as when viewed from Earth to apparently overlap into a filled, circular transmitting aperture. The single large synthetic aperture would serve as the reflection surface for many hundreds if not thousands of power beams. The order of several 10s of SPS mass of lunar derived glass components would be adequate to make the reflectors. Costs of the reflector system would be divided over many power beams. Segmented lunar reflectors decouple the problems of beam geometry, details of microwave sources, power levels and rectenna sizes on Earth. LPS is fundamentally different from SPS in these regards. The segmented antennas would sum up to a much larger area than is possible for antennas built in free space. The composite antenna can project better defined beams than could the much smaller monolithic transmitters on an SPS. The lunar power beams would resemble spotlight beams, as in geometric optics, except they would be much better controlled and confined. Antenna engineers are not usually concerned with the development of optics-like microwave systems even though the engineering and physics are very well understood (34). Relatively large and efficient reflective arrays have been constructed and operated. It is useful to review the primary features of such optics-like systems and how they differ from the antennas systems normally designed for radar and communications. A key parameter in determining the nature of the wave field propagating from a radiating aperture is the transition length or distance L = D2Dlw which separates the radiation fields and beam intensities into near field and far field regions, the former being adequately described by a superposition of geometric optics and Fresnel diffraction and the latter by Fraunhofer (plane wave) diffraction. D is aperture diameter and w is the wavelength. Virtually all radar and high gain communication systems operate in the far field region. In the near field region (R < L), the maximum optical path difference from a field point to the distributed points of the aperture plane exceeds several wavelengths regardless of whether the field point is on or off axis of the system. Under such conditions, the radiated intensity is effectively averaged or smoothed for lateral displacement within the geometric optic beam and rapidly attenuates beyond the beam edge. In contrast, the far field (Fraunhofer) region has a maximum optical path difference of less than a wavelength between an axial field point and all points of the aperture plane. The radiated intensity decreases with lateral excursion from the beam center (with or without undulations depending on details of the aperture excitation).
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