offer advantages which could greatly reduce the harmonic amplitude along with the bandwidth of the background noise. Details of amplitron filter design and performance are covered in a later section. The cold cathode for the amplitron should result in yet lower noise generation. Low noise and harmonics will be a major objective of the technology development program. 4. 1.8 PARAMETERS VERSUS FREQUENCY The baseline design of a 5 kW power gain amplitron operating with high efficiency at 2450 MHz was frequency scaled to four other frequency levels from 1.0 to 6. 0 GHz. The designs were then current scaled to give the estimated optimum power at each frequency. The main criterion was maximum power for the least overall system weight and cost. For scaling, the maximum temperature drop across the vane was held constant at 95°C (at 6. 0 GHz, 125°C was allowed). The parametric design was not carried above 6 GHz. The previous section on propagation has shown that rain attenuation makes the range above 6 GHz of little interest from a system point of view, and in addition: a. In a crossed-field device, the rf currents are on the microwave circuit so that the I2R losses increase with frequency. b. The structure size varies inversely with frequency, so the tube gets very small at X-band and above. For the same current density, therefore, output power per unit decreases. c. The small size of the tube would increase the cooling problem. Above approximately 6 GHz, the cathode would require some auxiliary liquid or vapor cooling. d. As the size decreases, the voltage gradients increase, and this increases the chance for arcing. The power levels investigated for each frequency are shown in Figure 4-4. This is the power added per tube. All have a 7 dB gain and will operate at 20 kV\ The specific weight and cost are shown in Figure 4-5. From these curves it is seen that the range from 1.5 to 2. 5 GHz should yield the least overall tube weight and cost. Radiator size and weight data are shown in Figures 4-6 and 4-7.
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