Miscellaneous Items Other considerations which should be included in the system design and evaluation are: Mutual coupling among the active elements which may have a pronounced effect on the phase control system and its behavior. These effects should be studied now. More studies should be undertaken to determine if phase control at the subarray, rather than the power module level would be sufficient. This could have some significant cost advantages. Ionospheric effects on the pilot beam are totally unknown, both in magnitude and in temporal behavior. These should be studied as soon as possible. Range measurements on the radiating elements will be hard or impossible to accomplish to the 1% accuracy level. Multipath effects are the major problem. However, it is the feeling of the panel that good range measurements are very important and are worth the investment early in the program. The scan time (1 min) of the closed loop system presented by Novar is probably too slow to provide the required control. Although there was not complete agreement, the panel tended to favor a closed- loop phase control system over the retrodirective approach. The onboard broadcast phase reference system presented by Rockwell in connection with the solid-state sandwich configuration seems appealing because of its freedom from ionospheric variations and interruptions. Work should proceed on both closed loop and open loop systems. None of the phase control systems presented are clearly superior at this time. 5. MICROWAVE AMPLIFIERS There is still no definite answer as to which choice is optimum for the microwave power amplifier devices. At this time the klystron looks most favorable, but either the solid-state or magnetron source may look better later. The question of optimum power transmission voltage and amplifier size should be very carefully studied and re-examined. Some attempt should be made early to determine the maximum voltage which can be safely used in the SPS environment, as this has a significant bearing on many design decisions. If it is not possible to operate at 40-50 kV, then klystrons cannot be used. Better ohmic efficiencies will be obtainable with higher voltages and the klystron efficiencies will increase as well. On the other hand, reliability of power conditioning equipment and the klystron tubes themselves will probably decrease as the voltage goes up. Some test of a high voltage array, perhaps piggyback on a SAMSO mission which is going to GEO, should be considered. Tests at LEO will not be satisfactory since the two plasma environments differ significantly. Moreover, the SPS itself may significantly alter the GEO plasma environment. Noise and Harmonics One of the major problems with any of the power amplifiers being considered is that of noise and harmonic generation. It appears most likely that the klystron will have much less noise than either the FET approach or the magnetron. The noise question should be answered at the device level first. Once a device has met the noise requirements, subarray tests should be started to look into the mutual coupling behavior. In as much as klystron characteristics do not scale well, no effort should be spent on developing a lower power klystron than that which is anticipated for actual use in the SPS.
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