Still another key factor to be considered is radio frequency allocation and radio frequency interference. As a general rule, the lower the frequency the more impact there would be on established users of the radio spectrum and the less economically attractive would be an SPS on a cost benefit basis. Specific ’’natural" frequencies to be avoided if possible are the space hydrogen and hydroxyl emission lines at 1.4 GHz and 1.7 GHz respectively, which are under continuous observation by the radio astronomy community. Although current and planned usage of the spectrum for space activities, including the NASA unified S-band from 2.1 GHz to 2.3 GHz and the communications satellites beginning at 3.7 GHz, could be shifted to other frequencies in the extended time frame needed for an SPS development, it would also be prudent to expedite MPTS development by avoiding these frequencies as well. The recommended choice is the industrial microwave band of 2.45 GHz ± 0.05 GHz. This is cost effective on both a system and DC-RF converter level, as previously shown, provides continuity with prior rectenna development, avoids potential frequency allocation problems, and minimizes impact on other spectrum users. Users of the spectrum from 2.4 GHz to 10 GHz with sensitive receivers would have to protect themselves with notch filters against the fundamental and up to third or fourth harmonic emissions. High gain antenna (60 dB) radio astronomy observers in view of an SPS would be denied only the basic" 2.4-2.5 GHz band if klystrons were utilized as the dc-rf converters. The higher projected noise output of amplitrons, even with filters, would exclude an additional range up to 2.7 GHz; and where the SPS were in the main lobe of a 60 dB antenna, would exclude observations above 1.9 GHz. The amplitron noise estimates are based on measurements of pulsed tubes and as such may be too high, since there is some evidence that continuous wave operation as proposed for MPTS may reduce noise considerably. The trend of MPTS characteristics at a design center of 2.45 GHz and 5 GW-10 GW power level is shown in Figure 48 for ampli- tron-aluminum configurations. SPS cost changes little at a given power level so that the taper-beam interception efficiency choice can be made on the basis of such factors as minimum ground power density, reduced land use, minimum antenna weight, etc. We can expect in fact that the modular designs described for the subsystems would lend themselves to configurations tailored for a particular site, with variations in the weight given to the key factors. The 5 GW, 5 dB taper, 90% beam interception case is chosen as the single baseline for further evaluation. Figure 49 provides a comparison of dc-rf converters and material choices for a 5 GW baseline system. We see that the klystron designs are substantially heavier and more costly as could be expected from the component characteristics provided earlier. For waveguide and structure, the graphite material choice reduces weight but has negligible impact on cost because of an offsetting increase in processing cost relative to aluminum; however, development of manufacturing and assembly concepts, especially for orbital use, could very well shift preference strongly to one or the other material.
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