ENVIRONMENTAL CONSIDERATIONS FOR THE MICROWAVE BEAM FROM A SOLAR POWER SATELLITE G. D. Arndt and L. Leopold Johnson Space Center Houston, Texas ABSTRACT Solar power satellite (SPS) systems in geosynchronous orbit are possible future energy sources. The SPS concept uses a highly focused microwave beam to transmit the energy to the earth. The microwave power transmission system parameters are summarized, emphasing those parameters which may affect the earth's environment. Environmental impacts of the microwave beam are discussed. The power levels for the main beam, sidelobes, grating lobes, and ground receiving antenna reradiation patterns are presented. lonospheric/microwave beam interaction studies, experimental and theoretical, are reviewed. Possible radio frequency interference sources and tne magnitude of expected interference levels are discussed. A number of studies using large solar power satellite (SPS) systems in geosynchronous orbit (GEO) to supply power for Earth use are now underway. This concept requires power transmission by a microwave beam from GEO to the Earth's surface. Each microwave beam provides approximately 5 GW of electrical power to the commercial utility grids. The power density levels associated with the microwave beam poses potential problems for radio frequency interference to existing communications and navigation systems as well as possible low- level radiation concerns for public health and ecological balance within areas close to the terrestrial receiving rectennas. This paper summarizes the microwave power transmission system parameters, with emphasis on those parameters directly affecting the Earth's environment. The antenna pattern characteristics for the main beam, sidelobes, and grating lobes, as well as the rectenna reradiation density levels are given. The results of initial studies into microwave beam interactions with the ionosphere are reviewed, together with possible problem areas. The noise characteristics of proposed DC-RF power converter tubes in their respective antenna configurations are discussed. The present SPS microwave system has a large, 1 KM diameter phased array which is divided into 7220 smaller subarrays, 10.4 meters X 10.4 meters on a side. The subarrays radiate through slotted waveguides with the DC-RF power converters mounted on the backside. Each subarray, with uniform amplitude and phase illumination across the surface, has its own phasing electronic and RF receiver to process a pilot beam from the ground; The subarrays are phased together to form a single beam at the ground rectenna. The ground rectenna illumination function has a diameter of approximately 10 KM and receives 88% of the transmitted energy. ANTENNA PATTERN CHARACTERISTICS The SPS microwave system must have a highly focused main beam v/ith low sidelobes and grating lobes. There should be minimal wandering of the beam boresight, with a fail-safe mechanism in the event of a total failure of the phase control system. An active, retrodirective phasing technique satisfies this latter requirement as will be shown later. A number of different antenna illumination functions were investigated to determine an optimun illumination in terms of maximizing the power in the main beam and minimizing the sidelobe levels (Ref. 1). The 10 dB Gaussian taper which had the best overall performance provides a power beam with a peak of 23 mw/cn/ at the center of the rectenna and 1.0 mw/cm^ at the edge as shown in figure 1. The first sidelobe has a.07 mw/cm^ peak at a distance of approximately 7.7 Km from the center of the rectenna. This sidelobe density is over a factor of 100 less than the present U.S. standard of 10 mw/cm? (but exceeds the USSR guideline of .01 mw/cm2). It is not anticipated that these sice- lobe levels will be an environmental problem: however, there are a number of new studies sponsored by the Department of Energy which will be addressing the allowable radiation levels. It is possible to further reduce the sidelobe levels by increasing the amount of Gaussian taper. Increasing tne taper produces a lower boresight density, a wider mainlobe, and lower sidelobes. A 17 dB taper can reduce the sidelobe peak to .01 mw/cm^, the USSR guideline (Ref. 2); however, this large taoer has greater thermal dissipation problems at the center of the transmit array due to waste heat from the power tubes. An active cooling system for each tube would probably.be required for this taper. l Pawar aeaeity at a* a a« team bar*M
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