SPS POWER TRANSMISSION - ARE MULTIPLE BEAMS A BETTER OPTION FOR EUROPE? Ross A. Henderson and John P. Stark British Aerospace Dynamics Group - Bristol UK The US baseline SPS Reference System dictates an electrical input to the utility grid, of 5 GV/ (gigawatts) transmitted from geostationary orbit via microwave energy. This power level is proved to be the optimum in relation to parameters such as the net cost of supplied electricity, transmit antenna size and benign ionospheric interaction of transmitted power energy. The addition of a 'safe1 microwave environment at the ground receiving site (rectenna) leads to very large intercept area which becomes disproportionally large at European lati tudes. The environmental, political, economic and social impact of these rectennas are enormous even if 'off-shore1 siting is feasible; however, the additional technical and operational problems imposed by these large centralised power utilities demand a new approach to the future planning of involved utility supply i ndustri es. A multi beam microwave transmission concept is proposed which has minimal impact on present SPS concepts. The rectenna areas are significantly reduced and can be related more closely to user country or 'community' requirements for electrical power needs. Utility grid costs and complexity are reduced; interface requirements being commensurate with present technology. In deriving alternative system configurations, it is necessary to ascertain those parameters which must not be varied, those that can be marginally varied, and those to which the system is relatively insensitive. In the first case come those parameters relating to the safety levels of the microwave beam and the thermal levels for the transmitting antenna. Thus the configurations considered maintain a peak flux density of 23 mW/cm2 in the beam and peak heat dissipation levels of 22 kW/m2 on the transmit antenna. Additionally, safety zones 1 and 2 are defined as having peak flux densities of 1 mW/cm2 and 0.02 mW/cm2 respectively. The transmitting frequency remains 2.45 GHz and the total power input to the ground utility system/satel1ite remains 5000 MW. Parameters dependently varied are the spacetenna and rectenna sizes, this dependence arises from the ionospheric peak flux, the heat dissipation at the transmitter, the total power in the beam and the transmission efficiency. If the transmitting antenna diameter is increased, a narrower beam can be produced (leading to smaller rectenna areas), but to keep the peak fluxes in the ionosphere below the above limits, the total power in the beam must be reduced. The efficiency of the beam transmission is dependent upon the product of the space and ground antenna areas and in developing the multiple beam concept this efficiency is kept invariant. There are several ways in which one might realise a multiple beam phased array antenna. Of these, the one which adheres most closely to the system requirements is by adoption of an aperture distribution which is configured from n identical distributions. Although the use of n superimposed phase distributions on each radiating element appears attractive, element mutual coupling could prove to be a severe problem.
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