to n/l-rj , where 7] is the device efficiency. The amplitron is predicted to be approximately 4 percent more efficient than the klystron. When comparing an 81 percent efficient klystron versus an 85 percent efficient amplitron, the above proportional factor causes the klystron to have approximately 25 percent more power to radiate or reject than the amplitron, assuming that all factors are equal except the 4 percent disparity in efficiency. Figure 7-46(a) is a plot of the proportional factor 77/1-17 for efficiencies ranging from 80 to 90 percent. As can be seen, a device which is 85 percent efficient can produce an output power 5. 67 times its dissipation, whereas a device which is 81 percent efficient can only produce 4. 26 times its dissipation. Figure 7-46(b) is a plot of the klystron output relative to an 85 percent efficient amplitron versus the klystron differential efficiency. In reality the difference would be smaller, since the klystron's geometry allows active cooling and its magnetics may run approximately 50° C hotter than the amplitron, which makes its heat radiator more efficient. A klystron system would generate more heat for a given power output and place more demand on the heat rejection subsystem. The amplitron has a potentially long life, since a pure metal secondary emitting cathode will be used for which there is no known life limitation. In addition, since the cathode is not heated, the overall construction of the system is simplified. The klystron, on the other hand, has a heated cathode and an active (heat pipe) cooling subsystem, which add complications and limited life components. The klystron amplifier has better radio frequency interference (RFI) characteristics than the amplitron. RFI characteristics are important since interference may result to communication channels and other related users of the radio frequency spectrum because of the very high power levels of the SPS microwave beam and its spurious noise and harmonics. The klystron is a linear device and has spurious noise of 90 dB relative to the fundamental, whereas the amplitron is a saturated amplifier with spurious noise of 40 dB relative to the fundamental, making the klystron more suitable at this time relative to RFI. 7. 3. 5 SUBARRAY SUBSYSTEM The 1 km transmitting antenna consists of 1652 20 x 23 m subarrays (Fig. 7-47), each individually adjustable by mechanical screw-jacks. The subarray consists of 230 basic elements (1 x 2 m). These elements are the lowest replaceable units (LRU) in the transmit array. The LRU was sized based on two primary considerations. First, the 2 m2 area allows one to eight amplitrons to be mounted on any LRU, which provides an eight-step power taper. Second, the LRU should be small enough to be handled by a man inside a repair module. The LRU is small enough to be removed or replaced from the rear of the
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