for aluminum waveguides. The klystrons are mounted behind the waveguides and radiate thermally outward via graphite radiators or heat pipes. However, the area between the waveguides and the thermal radiators for the klystrons will act as an oven and operate in the 450-485° temperature range. Therefore, a 1 km or larger transmit antenna is needed for a 5GW 10 dB Gaussian taper illumination. (b) Microwave power density limitations in the ionosphere - As the antenna array size increases, the maximum power density transmitted through the ionosphere increases. Previous studies (ref. 1, 3) indicate that non-linear interactions between the ionosphere and the power beam begin to occur at some threshold power density level which is dependent upon the operating frequency. This threshold level is 23 mw/cm^ for the model SPS system using an operating frequency of 2.45 GHz. This places a maximum size on the antenna for a given power output at the rectenna. The maximum power density at the rectenna as a function of transmit array size is given in Figure IV.A.2-2. Three output DC power levels, 1GW, 5GW, and 10GW, at the rectenna are shown, together with a 5 dB and 10 dB Gaussian taper for the 5GW system. These curves indicate that a 1 km array, 10 dB taper, 5GW system is at the maximum power density level set by the ionospheric interaction limit of 23 mw/cm^. However, this 23 mw/cm^ density level can only be considered a guideline, not an absolute requirement. There is not sufficient experimental data available to accurately predict the exact threshold level. (c) Maximum output power - If Figures IV.A.2-1 and -2 are superimposed such that the power density limits coincide (Figure IV.A.2 -3), it may readily be determined whether a given combination of output power and transmitting antenna diameter exceed either of the two limits. For example, 6GW output and 0.8 km diameter fall within the ionosphere limit but greatly exceed the transmitting antenna thermal limit. Since the only issue is whether an operating point falls above or below the limit line, the relative sizes of the vertical scales are unimportant for this superposition. For the limits used here, the maximum output power is 5GW and the corresponding antenna diameter is 1.0 km. A high output power has a number of economic and operational advantages discussed elsewhere in this report; consequently, the maximum of 5GW output power, together with 1.0 km transmitting antenna diameter, has been used for sizing purposes throughout this study. These results will change if the maximum power densities are revised. For example, if the transmitter limit were 25 kw/m? and the ionosphere limit 40 mw/crrr, the maximum output power would be 7.1GW and the antenna diameter 1.1 km. (d) Rectenna size - As the transmit array size increases, the beamwidth of the main lobe decreases in proportion to the array area, which causes the rectenna size (and cost) to decrease. The tradeoffs of
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