Microwave Design", that the klystron amplifier is the "cleanest" tube at frequencies above and below the fundamental pass-band frequencies. The magnetron is rather noisy near the fundamental frequencies and frequently requires filtering. Amplitrons and magnetrons have the least second harmonic output, whereas the TWT's have the most. Magnetrons have particularly high third harmonic output levels. Klystrons and magnetrons generally work well into reflective filters, although absorptive filtering is recommended at megawatt pulsed or kilowatt cw (continuous wave) power levels. If one considers the crossed-field devices (particularly the amplitron) because of parameters such as high efficiency and minimum weight as have nearly all present studies for proposed SPS designs, the following information should be noted. Most reentrant crossed-field amplifiers have cold cathodes and are started by applying RF drive. The RF drive power must be applied in time to permit the tube to start drawing current before the cathode voltage pulse overshoots the proper operating voltage. However, even when there is a drift region, the tube does not stop when RF drive is removed. The reentrant electrons still carry enough energy so that secondary emission from the cathode is maintained, and the tube will oscillate near a band edge or generate broadband noise until the cathode voltage pulse ends. From the above information, it is concluded that the amplitron generates broadband noise when RF drive is applied or removed until the tube stabilizes. Figure IV-C-2-b-l illustrates this unstable condition during application or removal of RF drive. The amplitron and the klystron will require extensive testing to determine the exact noise characteristics for cw operation and to determine the degree of suppression or filtering that will be needed. (3) ANTENNA CHARACTERISTICS AND RFI The antenna being considered for forming the SPS microwave power beam directed toward the earth is a phased array, which makes use of the phase relationships among a number of individual radiating elements for combining the power from each element to form a main beam. The critical phase relationship between elements is a function of frequency as well as separation distance and the relationship of these parameters affects the main beam and sidelobes. For RFI considerations, the sidelobes at the operating frequency and off-frequency (spurious radiations and harmonics) are of interest. It should be noted that even with a clear channel assigned to the SPS, it is possible that interference may result to communications spectrum users due to the spurious responses of their individual equipments. Unfortunately, no information on the off-frequency characteristics of phased array antennas could be located during the period of this study. This likely results from the past applications of this type antenna to radar systems, where off-frequency performance is not a primary consideration and is usually tolerated. Considerable tests will be necessary to develop this information for the SPS antenna application to allow evaluation of power station spurious and harmonic radiation effects on other users of the frequency spectrum.
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