Table 6-11 Transmission Frequency Sensitivity by Case The reason for this is that for the same beam slewing capability, the transmitter device size/spacing is larger at 2.45 GHz than it is at 5.8 GHz by approximately the inverse ratio of the two frequencies. Since both the power and cost per device has been assumed to be the same at both frequencies, the same number of devices would be needed to produce an equivalent output power and the array costs would be approximately the same, even though one was much larger than the other. Also, the larger array required by the 2.45 GHz system, has the effect of narrowing the normally wider beam produced at the lower frequency, and this results in rectenna diameters that are not much larger than those required at 5.8 GHz. If there’s a penalty to be paid for using 5.8 GHz rather than 2.45 GHz as the transmitter frequency, it is in the susceptibility to rain. Figure 6-9 graphically depicts the variation in the power delivered by a MEO SunTower Figure 6-9 Delivered Power Sensitivity to Rain Rate system at both frequencies, as a function of the rain rate at the receiving site. In this figure, the frequency penalty appears to be more severe than it actually is. It is seen that the delivered power is down about 40% or a little more than 2 db at a rain rate of 5 inches per hour, but that would be a torrential downpour that is highly improbable at most sites and unlikely to last very long if it does occur. A heavy rain would be one inch per hour, and at that rate the delivered power would be down only about 1/3 db. Based on this data and the results given above, the frequency preference of 5.8 GHz would appear to be justified. 6.2.2 . Beam Steering Capability The impact of beam steering and its angular limits is manifested in different ways among the three concepts of Cases 4, 13 and 16. This is illustrated by the three charts in Figure 6-2 which indicate the unique ways that beam steering benefits each of the three different concepts. The SolarDisc concept of Case 4 is the only one of the three for which an electronic beam steering capability is not essential In GEO, a SolarDisc can simply transmit all its collected energy to a single ground site within its limited FOV, as was proposed for the 1979 Reference System. However, as shown by the uppermost graph in Figure 6-10 , even a small beam steering capability provides the SolarDisc access to many more ground sites and opens the market for more potential customers who could benefit from smaller amounts of time- shared power. Although the number of sites shown in the chart may not be impressive, it should be
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